Making clay buttons is not as easy as it sounds. I am a potter and a friend of mine made buttons from stone wear clay. You make the buttons, fire them in a kiln at very high temperatures, glaze them and then fire them again. There are certain types of clay that you can fire in your oven or air dry them but I am not sure how strong the button will be since i never worked with that tupe of clay. If it is for a wall hanging, not a problem, but for a quilt to be used and washed, I just do not know if these types of buttons would hold.
I saw some nice buttons sold by ACMoore, the craft store. Maybe you can try there, or estate sales, garage sales, and of course the eBay. Good luck.