Before you get started, I suggest you read some of the forums on here and get some info on starting up a tank and the different fish choices and set up ideas. Armed with a bit of knowledge you can determine which fish stores are good and which ones are not.
I am not from Colorado, but if no one comes back with a recommendation, you can always shop online. LiveAquaria.com seems to have a good reputation. I am sure there are others. You can probably do a yellow pages search for fish stores/ petstores in some of the major cities and go visit. That is how I found some that are not too far from me. Good luck.