http://www.paintscratch.com/ will work if you just want touch up paint...they do sell gallons but it ain't cheap.
If you want to repaint the whole car then any automotive paint store can hook you up...you just look up the paint code and they mix it up for you. Heck, even Napa can do that (though the paint isn't the best).
Prices will vary, I had 48 dollars in my supplies when I painted my car...I will say it will cost you ~250 for paint, primer, and small supplies.
Stinger