Local fish stores in CO are a little sparse and it seems like half of them have gone out of business, but there are a few that are pretty good in my opinion. One of our favorites is Fish Palace in Aurora. They definitely pull in some pretty cool cats there! We've gotten some of our favorites there including a tig, achara, and planiceps. Typically they also have rtc, tsn, and rtcxtsn as well. They've had a gulper cat there for a while and from time to time they get various other pretty unique cats. They also stock some other really cool predators that are harder to find at other stores. Be prepared to pay for it though 'cause they aren't cheap...not overpriced necessarily though. They just stock pricey fish. Another thing that's nice about Fish Palace is that if you're looking for a specific fish if they can find it and order it for you, they often will. A lot of the stores around here that I've been to won't do that.
We also like About Fish Inc. off of Federal Blvd in Westminster. They don't carry as many of the hard to find predators, but their fish are healthy and their prices are good. They typically have a decent supply of cichlids and we got a very healthy 4" silver aro from them for $29.99 which we thought was pretty darn reasonable. They do a bit of breeding there too, so if you're looking to grow your cichlids out or buy a bunch to find a breeding pair they've got you covered. They also do some equipment consignment there. We found two nice acrylic tanks with filters and stands at very reasonable prices, but of course, the consignment stuff is hit or miss.
Aqua Imports in Boulder is usually worth the trip. They carry a wide variety of fresh water fish and occasionally bring in the rare and unusual. In my opinion, their prices are a bit inflated, but if it's the right fish it could be worth it...
Golden Fish off of Colfax in Lakewood is good for flowerhorns, discus, and the occasional hard to find predator.
Hope that helps! By the way, I think Denizens of the Deep is now a store called Great White Aquatics. They periodically post new info. about their stock on fort collins CL.