I think , with Innova anyways, that the custom hot stamps are all done in house. You submit the artwork, they stamp it on the discs of your choice.
Mark Molner at Disc Golf Values would be the guy to ask.
You should be able to find a few people in your area that can custom dye discs for you, or you could just dye them yourself.
yeah, the other thing to consider if you wanted to do stamping yourself, is that the companies are VERY reluctant to let any unstamped discs go out the door. You can get some here and there, but it'd be nigh on impossible to get any in a large quantity. You'd have to get factory stamps, which wouldn't be very cost effective in the long run.