I've had die cutters for years and have had many custom dies made at various places. They are actually called 'Steel Rule Dies'. Never heard them refered to as 'knife dies'.
I've had them made by Accucut, Ellison:
http://www.ellisoneducation.com/customdies
I've also had dies made by Cross-Cuts, but it doesn't look like they are doing custom work anymore.
Actually, your local printer can probably tell you where to get a steel rule die made locally. The problem is that commercial printer's dies don't have a full foam layer and would not be suitable for use with fabric. You could make it work, but it would be a pain.
PM me if you have more questions.