Do you have one to replace? If so the first place to check is the ballast OEM. IF they don't have it, try IceCap (I think they used to repair ballasts and have a decent stock). McMaster is also a good place to look.
The electronics supply houses (Allied, digikey, Farnell, Newark, etc) will have them, but they will be pricey for what you are looking for.
If you post the specs, we can help you find one (hopefully for a few bucks). You need to at leat match the voltage rating on the cap, but higher is fine. The cpacitance rating should be within 10% or so of what you are replacing. The sad joke is most cheap caps that are found in these units are 20% tolerance (and some as bad as 50%).
Bean