Custom fabric buttons are easy to make and kits are commonly sold in all major fabric stores, sewing supply companies, and Wal-Mart for $3-5. You can also buy them online.
We used a lovely kit for ours made by Armommy
which is a basic fat quarter. You can make your own by tracing out 1 1/2-2 1/2 inches each on any fabric of your choice. The white plastic guide is included in your kit.
You will need:
plastic press and form (white and blue pieces)