For these, I usually like 1 inch buttons. It is totally ridiculous to pay $3 or $4 for just 12 buttons at stores like Wal-Mart when I can buy several hundred for the same price.
I have 3 places I check when I am ready to re-stock my button-stash: Oriental Trading, S&S Worldwide, and Sunshine Discount Crafts. Links are below.
Just go to any of them, do a search for buttons, and you will be so happy you did. The selection differs from time to time, but between the 3 sites, I can almost guarantee I will find one place to buy 200 to 400 for under $5.
With all these buttons, you can do everything from towel toppers to jewelry and home decor trims. Have fun.