Any electric supply store. getting it locally will save on shipping. So you end up supporting your local economy.
However, if you're looking to replace a defective unit you own. (already own the endcaps, reflector, ect) then just get the ballast. If you're looking to get all you need- ballast, wiring, lamps, housing, you might be better off getting a package deal. Like at AHsupply.com or another place. Do a search for ballast and check out what you find.
HTH
B