Have you been to Todd Schlafer’s garden? That is by far the best option in my opinion. He has much more than just Rocky Mountain yamadori also.
I believe Harold Sasaki has a garden called Colorado Bonsai and also sells trees but never been myself.
If you are a member of RMBS, there are several members who sell trees also.
To be honest the options are pretty limited given the size of the city and location relative to the Rockies, you’d think there would be more options.
I also always have a few pre-bonsai for sale.