My clear buttons are by 7gypsies from quite a few years ago, but you can find them through other companies, too. Just do an internet search for "clear buttons".
When stamping on clear buttons or anything non-porous, you must use a solvent ink like Staz-On Ink by Tsukineko.
Lay the button down gently on the stamp. Press and lift off when image has been stamped.
Remember that you'll need to stamp on the FRONT side of the button if you are stamping letters or words.
Try layering a patterned paper behind the stamp. Adhere paper to button with dimensional glue and trim off edges.
And voila! A totally unique embellishment!