What defines "relatively inexpensive?" And what kind of riding will you be doing?
The Front Range has no dearth of bike shops. Most of them will be able to point you in the right direction. I'm not too familiar with Denver, but Wheatridge Cyclery in Wheatridge and Boulder Cycle Sport and Sports Garage in Boulder are all great. Sports Garage tends to carry higher end stuff and is a little pricey though. Most other shops will have a wider variety. The Boulder and Denver Craigslists are always packed with bikes too, so it's just a matter of finding something that's suited for your riding preferences, fits you, is in good condition, and is not overpriced.