Mister Coaster,
Try Craigslist
http://denver.craigslist.org/pet/
You can find tons of deals from this site. Just remember to find out how old the aquarium is , was it ever stored dry and for how long, and if you can, a picture of the aquarium when it was set up. Silicone gets brittle over time and this process is sped up when the aquarium had been stored dry for over a year. Getting a pi of how the tank looked like set up will (hopefully) give you an idea if the tank was level. A long tank that is out 1/8" at one side can be out up to 1/2" on the other. This puts a lot of strain on the glass and silicone seals. Check the cabinet for water damage - warped wood or damaged finishes will indicate a leaky tank and finally, fill the tank with water on a dry, level surface. A leak will show up pretty fast. Don't get a used tank that had reptiles in it! It could have been specifically designed as a terrarium (thinner glass and silicone) or the critters themselves have damaged the silicone. Finally, most of the cost in a new aquarium is is the cabinet and the equipment. Aquariums themselves are relatively inexpensive when compared to the cabinet, lighting, filtration and decorations. Getting the equipment used and buying a brand new tank may save you costly water damage fixes!
Other spots to check for used equipment - local aquarium societies, mom & pop pet shops, aquarium forums on the internet.
Good hunting!