I recently acquired an antique lamp that is powered by a mogul base incandescent bulb and a 32w T10 circline. As I was inspecting the electrical condition I found it had 2 wax capacitors.

One directly across the 120v AC line and another across the manual preheat switch (normally open unless preheating it).

Where can I buy some power capacitors for AC line usage? I've heard that I need to use X or Y caps but I am unsure of which one to use to replace them.

Thanks

The "X" rating means itisto be connected between main live power conductors (phase vs Neutral, between phases,...). Requirement for that capacitor asks for mainly a self recovery feature (or extra overvoltage robustness) and overcurrent protection (in case it fails so, the self recovery won't switch that off)

The "Y" rating means the use between some live terminal and protective earth terminal. Technical requirement means mainlythe leakagecurrent introduced bythat capacitor to be limited below a limit (most frequent standards state 1mA as sum from all used capacitors, if I remember well) and it shall not deteriorate even when exposed to an overvoltage. The leakage limit means the capacitance is limited to just few nF.
"Y1" means the design does not guaranteesufficient impedance after some single failure, so it could be used either only on earthed devices (so class I), or in series pairs (so even when one fails short circuit, the second still limits the curent).
"Y2" means the design is made so, no single point failure could lead to the leakage current becoming above the limit (practiccally it means a design with two capacitors in series in one package, but arranged so no crack could propagate to affect the dielectric strength across the main terminals), so suitable for insulation class II devices.

So for the connection across mains input you need "X" rating.
The capacitor across the starter teminal is not directly acrossthe mains input (in case it fails the currentis limitedby the balast), so does not need any such rating. But the functionality would ask for at least 1kV DC ratingwhen the capacitor does not feature any self recovery, or 630VDC with self recovery feature.

I would look on line.
Mouser Electronics has just about every kind of capacitor ever made.
If you find it difficult to find the one you want in their catalog, you can call them and explain to them what you are looking for and usually they can cross reference it to a newer one.
I have bought a lot of electronic parts from them over the years with good results.  

For the capacitor parallel to the starter switch I would choose ~3..15nF.

The one across the line input isn't necessary for the ballast operation (if it is really directly connected between the Phase and Neutral), it just correct the power factor, so to reduce the mains current. And that is in home environment practically not needed with such low currents.
Why I ask:
But what type of ballast is used there? An autotransfdormer? That sounds rather odd for a manual preheat.
What I would suspect is a series LC reactor ballast: The capacitor has higher impedance than the coil, but the close-to-resonance operation ensures there is enough OCV during arc reignition. This type of ballast need either an electronic starter, or a manual start switch (it won't work with a glowbottle). This topology is the most efficient "nonelectronic" topology I know about, mainly when the arc voltage is just close below the mains voltage (it works for arc voltages up to ~80% of the mains).
But with this ballast style the capacitor is an essential component of the ballasting impedance, so it's capacitance should be exact. But that should match the exact choke, so each ballast model may use different value for the same lamp...
What I know, the voltage rating of this capacitor should be at least 220..250VAC (for 120V mains), but you have to tell the capacitance.
"230V" power factor correction capacitors would work here.

So the only capacitor I really need is the one parallel to the starter switch? I shouldn't just get a similar value to the original, (smaller one in pic), which is .006mfd? And I'm assuming mfd=μF?

Also did you mean ~3.15nF? or does the extra period mean something?

I'm also not really sure what type of ballast is in it. It does have 3 wires coming out of it which seems to suggest that the voltage has to be stepped up for the circline's operation.

I also noticed the cap across the mains voltage (bigger one in picture) says ".02 MFD. -002-.002 MFD" and I dont really know what that means.

It means a value of a cap  2 hundreth of a micro Fards

The "double period" in means a range, so "3..15nF" means anything between 3nF and 15nF...
On the phoo on the cylinder I see 6nF, which is in that range what I would expect.
The other (box) capacitor seems to be 20nF, so probably just some kind of RF supression capacitor.
The text "-.002-.002MFD" behind the ".02MFD" is strange. Is the case connected somewhere? If yes (I would expect grounded,...), the boxy capacitor could be a combination of a 20nF "X" capacitor (so between Phase and Neutral) and two 2nF "Y" capacitors (one connected from Phase to Ground, second from Neutral toGround). It the past such combinations were quite popular, you can replace it by those three separate components (look for "Suppression capacitors" on Mouser or similar distributors).

This should work for the smaller cap if you can figure out how to insulate the leads or otherwise make them inaccessible.

Would these also work? http://www.ebay.com/itm/X2-Polypropylene-safety-capacitors-472-0-0047uF-4-7nF-275VAC-k-10PCS-/251025104485?pt=LH_DefaultDomain_0&hash=item3a72431e65
They are a bit lower on the farad value compared to the originals

I'd rather have X2 capacitors for safety but I'm not sure how reliable ebay sellers are, especially from china  ... I looked on mouser for X2 safety caps but I couldn't find any in the right value.

If anyone else has any recommendations please let me know.

On a side note, would these same capacitors work in glow starters that needed to have their wax caps snipped out?

According to Medved, that value would work. It wouldn't surprise me if the same caps could be used in glow starters, but they look like they'd be too big to fit in a starter case.

For the mains filter, the "http://www.ebay.com/itm/X2-Polypropylene-safety-capacitors-472-0-0047uF-4-7nF-275VAC-k-10PCS-/251025104485?pt=LH_DefaultDomain_0&hash=item3a72431e65 " will work. If you want higher capacitance, put just 4 in parallel, you will be about at the original.
For the starter switch the problem is, the inductive kick goes into kV range, mainly with EOL lamps. And the capacitor has to survive it, therefore the kV rating.
But on the other hand even in a cold electrode dischatrge mode, the lamp does limit the voltage somehow and when there is no lamp, the circuit is disconnected, so I think even when it may degrade slowly, the "275VAC X" rated one will have reasonable life too, so I think it could be used there.

So you use one parallel to the starter switch and four parallel to the mains input.


With the glowbottle starters the "275VAC X rated" would suffice, as the glowbottle tend to limit the voltage.

I did some more searching about which capacitors to buy and I found http://www.mouser.com/ProductDetail/Vishay-BC-Components/VY2682M59Y5US63V7/?qs=sGAEpiMZZMt1mVBmZSXTPHk%2f0R8m4oTi1Yau5yF0vSU%3d and http://www.mouser.com/ProductDetail/Kemet/C967U682MYVDDAWL35/?qs=sGAEpiMZZMt1mVBmZSXTPJQTuJwJyyFSLbwQJZEVz8I%3d , which should be more reliable than the ones on ebay.

But after I did some more searching I came across this article http://www.allaboutcircuits.com/vol_4/chpt_4/2.html which talks about "snubber" circuits to prevent contact arcing. I noticed they were saying that a resistor is needed in addition to a capacitor.

"If the resistor were not there, the capacitor might actually make the arcing during contact closure worse than the arcing during contact opening without a capacitor!"

Can anyone comment on this as to what value of a resistor is needed and why was this not implemented in the first place?

That resistor is needed with way higher value capacitors, e.g. 100nF..1uF, used sometimes with motor switches. It is necessary mainly in combination with thyristor and triac based switches, where the turn-ON can not be controlled (you just fire the component gate to turn it ON), while they are still sensitive to high current spikes. Mainly triacs are known for such sensitivity, but e.g. the GTO's in the electronic starters do not tolerate even the few nF normally used with glowbottles (tried, I have already destroyed ~4..5 Philips S2e starters in that way).

With the capacitances of no more than 10nF and mechanical switch you are safe without the resistor.
Don't forget the resistor makes the turn OFF action worse, as it allow abrupt voltage rise there (R*Iballast) and that may already start the arc.
And with the starting switches the main problem isn't as much the switch life, but it's ability to generate HV kick. That is the reason for the snubber capacitor there. Without this the energy stored in the ballast inductance would be burned on the arc across the contact, without generating any HV pulse for the lamp ignition.

eBay has the best prices on caps if you're not buying in bulk.