I found my saddle at Once Upon A Horse in Castle Rock. The gal was very nice to work with and helpful, and they had quite a few used saddles! I give them a thumbs up (and 4 mos later am VERY pleased with my saddle).
Not sure about tack, but our honeymoon was in Colorado, and on the way to Estes Park in the rented convertible, my wife screamed "GET OFF THE HIGHWAY NOW!!!" I thought "Jesus, a plane crash..." saw my hole and darted over 3 lanes to the offramp.
She had just seen Sheplers... I got to try on and fall in love with Ariat Ropers, and she got a pair of Cobalt paddock boots. We proceed to break them in all over the Rockies