I've generally had good results with eBay electronic parts and components sellers.

Just warning, be carefull. Some of the stuff sold on Ebay is fake

I order quite a lot of capacitors there for PC repairs (electrolitics). There are some capacitors where i can tell straight from the posted picture that they are fake, but a couple times i ordered and found out they are fake when they arrived (by closer look at them)

I saw a Mercury ballast that requires a 15uf cap in series with the lamp. The cap it is with has a big dent on the side. Do I need a new one? Could I use a motor start metal can cap since it's rated for 450?

The capacitor have a roll of foils & isolation inside, It is usually not right up to the walls though. If the dent is very light it does not reach the roll (only the filler), but if it is somewhat deeper it might (it might put pressure on the roll, making it start failing in that spot, and if it is a series capacitor in a ballast it might damage the ballast or lamp)

A motor run capacitor if it is the good type (not looking like an "electrolitic without polarity"), and has the right or higher voltage rating, i think will do fine in a ballast

But test old capacitors before you put them to use. Some are worn out and have much lower capacity then rated, some are beginning to break down and have leakage resistance parallel to the capacitor forming (few have a resistor in parallel put there intentionally, there the value of resistor is stated on the capacitor). I use a Fluke F115 to test them

It was a series cap. The ballot has been sold to someone else. I hope for the sake of the lamp and ballast they get a new one.

I think that any capacitor connected across the mains should be rated at at least 1 Kv so it can cope with mainsborne voltage spikes which we are told do happen although I think they'd are rare events its not worth risking using some lower rates cap. I think also that the cap might be there to stop the choke surge from getting to the filament lamp in your fixture

In 120 volt land, 630 or even as low as 450 is good enough.

120V or 240V rated capacitors can handle momentary peaks to much higher than that, thats not the problem unless they get them really too often

Capacitors will degrade just from leakages in bad spots in the isolating film (due to the imperfections causing spots with concentrated leakage, that spots just degrade with time even under normal voltage). To slow this degradation down by a BIG time, double the rated voltage is well enough so 240V ones will do for 120V

And an incandescent lamp won't really care about the kick from a choke (unless it is of really very big wattage)

I think that any capacitor connected across the mains should be rated at at least 1 Kv ...

The voltage rating lone is not sufficient for use directly across the mains input. Most materials can handle way higher than rather voltage for a short time, so 1kV overshoot could be handled well even by a 400V capacitor, but only for very limited time. The reason is, with the high voltage a good quality solid insulator needs time to develop the conductive path for the breakdown. It is called "Time Dependent Dielectric Breakdown", it is highly accelerated by temperature. So what sustains even 4kV 1-minute test may fail after few 1000 hours at 500V at 27degC, but just after barely 60hours when the temperature goes to 70degC.
Plus the periodic c harge/discharge on an AC circuit makes the thing even worse.

And whatever would be the rating, the capacitor may fail on it's own and then way other features are way more important to prevent huge problems like fire or so, in that view the smoke being just "an inconvenience". So beside the functionality robustness, the device should be guaranteed to fail in a safe manner, that is usually the weakest part of the cheepeese production.

So you should only use capacitors rated for the direct mains connection use ("X" rated), where the "X275VAC50Hz" with corresponding mark of a safety certification authority actually means way stronger and safer capacitor than just a "1kVAC/2kVDC"...

Could I use a HVAC cap for Power factor correction?

Could I use a HVAC cap for Power factor correction?

It depends, how it was used there. If it was motor capacitor, it may lack the guarantee it will not end up in flames when failing on hard connected mains, so it will require extra fusing.
Plus if it is just motor start (disconnected after the motor speeds up), it won't be able to handle the long term mains connection (usually the non-polarized electrolytic, but many film design as well; usually very small for the capacitance and voltage rating, but intended for operation just few seconds within a hour or so cycle), so not usable at all.

You may use it only if it is really a power factor correction cap, so connected directly across the mains (e.g. when the motor is running,...).

I'll check if it can be used across mains.

Also try Digikey.com they have fast delivery of genuine goods, or you could always rip open an old PC power supply.. 

In our case the capacitor is series with the ballast, so do the X2 requirements apply ? I think what is good for motor run is good for here