These are a few merchants that I use and are very reputable. I use them and refer them with complete confidence!

For used gear

KEH really does set the bar very high for used equipment. Everything I have ever ordered has exceeded my expectations. They also warranty their equipment so that you have peace of mind when you order from them. Definitely worth a look.
www.keh.com

A great local retailer for both used and new equipment.
www.thecameratrader.com

For new gear

One of Denver’s greatest photo stores for both digital and film equipment. Be careful, you’ll feel like a kid in a candy store.
www.denverprophoto.com

B and H Photo is the best on line source for gear of all kinds.
You name it and they have it!
www.bhphotovideo.com