Hobby Lobby sells them in a tube for a great price - especially when you use a coupon!
Also sometimes you can score a ton on ebay, Oriental Trading, thrift stores, Craigslist...
I remember getting them in a 1 pound bag at AC Moore as well. I'm sure Michaels and the other craft stores sell them like that too - by the pound.
I've been lucky that my mother volunteers at a rummage sale and has scored me bags of buttons... I don't think I'll need another button (unless it is a novelty one) for a long time!