Anyone know where I can get a few rolls of the solid core wire for fluorescent ballasts? Thinking of a project where I would need some.

man, I just changed out 5 fluorescent ballast 3 weeks ago. If only I would have had known you needed The Wire I would have cut them all off the old ballas and gave them to you.

 You can buy the wire from electrical supply house.   The use them
For ballast leads,   Low voltage control wire. 

Just search eBay for fixture wire #18

Try searching for TEW 18 gauge solid core wire. It should be 600v rated which should work with most fluorescent ballasts.

DigiKey has a decent assortment if you can't find a cheaper source for it.

Stranded wire technically does work, but the push in connectors on the modern lampholders don't play nice with it unless you tin the ends. Just avoid using automotive primary wire, they're usually rated for voltages found in vehicle electrical systems instead of 600v. 

McMaster-Carr sells rolls of it in various lengths and colors. Part no. 8428T4. I purchased black and white 25 ft rolls a few years ago. Here's the stuff I got.

I agree McMaster Carr is a good source for this sort of stuff too. And also for any special fasteners/electrical supplies/capacitors you might need for your lighting projects.

Their ordering is a bit confusing to those not used to their system though. IIRC they usually have this thing were they ship items out first and then figure out how much to charge you for shipping. Great for those in their target market like plant managers who might need items fast and don't have to woorry about costs, not as good for individuals/hobbyists that want to know the final costs of things before ordering.