Also, these two are in Denver and just a couple blocks away from each other:

http://www.fahrenheitsbooks.com/

http://mutinycafe.com/the-store

Fahrenheit has a pretty solid selection of collectibles, The Mutiny Disinformation Cafe doesn't have as many collectibles, but the staff is great and there are some good deals to be had. I found a copy of the S/L Dark Tower: The Complete Concordance for $35. Unfortunately it didn't have it's original slipcase, but it was in great shape, and the guy behind the counter threw in an $8 Raymond Carver book for free.