Amazon has them sometimes, but I'm not sure where else. You'll probably want to shop around the 'Net. If there's a Lego store in your area, they might sell cases as well as individual packs (I don't know, since there's no Lego Stores anywhere near me).

I know other people have done it, but I have never bought an entire case of minifigs at once. Like I said earlier, I like the challenge. :)