I think it’s called Versafine. Where can I buy them?

Stampeaz.com

Cheaper than at my local craft stores, more colors than at my local craft stores, and probably fresher, too.

I see some at Michael's.

Michaels & Hobby Lobby have black and sometimes brown, but that's all.

Sometimes Joann Fabrics online has them on a great sale and you can pick up a bunch of different colors also. That is the last time I stocked up on them 50% off sale