There's a couple of places that I've come across:
http://www.sshmi.com/index.html
www.grassburr.com
www.buybrownholsters.com
They show you a couple of different styles.
We've made a couple of them - mostly where they fit into the center console or in the door. With the holster being molded to the gun, we've not had any reports of the gun being "lost" in motion. Both kydex and leather would work well in this kind of application IMO.
A couple of considerations in your design might be:
- ease of grip (how quickly and easily can you get a grip on the gun).
- does it need to be concealed while in the vehicle.
- location within the vehicle (how easy is it for you to get to it when you need it and whether or not you want others having that same access).
- how is the holster itself mounted to/within the vehicle (a small consideration would be the potential resale of the vehicle - not too many folks are going to be interested if you've got 1/2" holes drilled where they normally would not be
)
Good luck to you and let us know how it develops.