I know I saw one partucular one...it was ALL kydex, maybe not intended for under desk, but could be. i think it was sold as one that can be mounted in a backpak, a car, amost anywhere.
I forgot to bookmark the site, I know I saw it somewhere, prob here.
there is always the option of buying kydex sheets and making your own. mock the holster up on a piece of plywood form the holster and screw it to your desk.
I've been by Red Hat's place, and he showed me a kydex under desk holster that he made - nice little setup. Let me just tell you - Red Hat is one talented man - He can make just about anything!
Funny! I just mounted a High Noon Bare Asset under my desk like an hour ago. I just stuck some velcro in the outside of the clip and my Glock 26 is sitting there nicely minding it's own business.
Most any holster can be screwed to the desk. I used a clip that was made to hold a table cloth to a picnic table. It holds a .25 auto nicely. Yes I know its only a 25 but rather than throw it in the trash I put it under the desk to get it out of my sight.
I had a Beretta stashed under my office desk for the last 14 years, before retiring in December. I never had to pull it, but stuck it here as my office partner was shot in his office ten years earlier in a hold up, and we both learned a lesson from that.