The site below handles shirt, suit, covered button few others.

This site has some unique buttons: http://www.buttondrawer.com/Todays_Features.asp

JoAnn's have sales, also try a few Thrift Stores, have bought a few things in the thrift stores just for the buttons high end rhinestone buttons...

Here is a link that might be useful: Buttons

Kari, if you are seeking donations, try posting in the "wanted" section of your local CraigsList. My parents generation saved buttons and some of us "inherited" the button jars. Since fewer people sew these days, they may be willing to part with them.

You can also buy buttons by the pound. There are sites on line but also check stores where you live. Sears used to sell them by the 2 pound bag, years ago when they still sold fabric. I bought 6 pounds and I'm still using them. The trouble is that often none of them match what I'm sewing. Over the years though I have used enough of them to save plenty over buying them by the card.