I used to do a lot of stamping for signature swaps. I used Fabrico ink pads. for use on fabrics and archival safe. permanent. I always heat set mine with the iron too, just in case. They changed their name but here are some
http://www.stampandgo.com/ACCESSINKPADSpage10.html can't find these easily. Hobby Lobby used to carry them but don't seem to do any other than for paper these days.