You can make them out of Fimo or other crafters clay. The clay comes in all sort of very bright colors, so you can design them as you wish. Then poke holes in them with a tapestry needle, or small straws, or skewer, etc. Then bake as directed.
There is a great button book called 50 Heirloom Buttons which I bought a few years ago when it first came out for about $15. It is now a collectors' item and goes for about $100! There's another one I don't know, but is supposed to be good, called The Button Maker.
And as pinksugar said, you can sometimes find things on clothing at Goodwill and just cut off the buttons and then cut up the coat or whatever for rags, so as not to waste it.