Don't buy these, make them yourself! So easy and cheap, especially if you already have pieces of fabric and buttons lying around the house.
Wooden disks for ten cents and Sharpies to write on them. The markers might bled a bit, but by varying how hard you press, you can control the bleed somewhat. Applying lighter colors before sketching any black lines works best, or else the black bleeds badly. Applying a light coat of spray acrylic before drawing also stops future bleeding, but it will turn the wood a couple shades darker.