Mark 23 Holster
I own a Mark also and have to say excellent purchase. It's the best/most fun shooting pistol I own, despite it's size and weight. If you are still looking for a holster I would recommend a Blackhawk Omega IV Ultra (maybe Elite, I own both and can't remember which is which). It is a tactical thigh holster and works very well with my Mark. It does tend to travel forward a bit and you'll notice some swing when walking and moving around a lot, but I can't complain. It'll run you around $100-125.