The felt that is used normally, is a compressed felt as said, you can buy it from saddle makers.

But craft felt works just as well, when glued to a flat substrate, wood (MDF) or craft foam core for sprays.

If you just want to experiment craft felt glued to foam core board is the way to go. All you are looking for is something flat that will hold the abrasive.

Paper and fabric also work just as well for sprays. SRD sells replacement felt strops for 9 bucks.