In Denver, and Colorado, it is not that common. You might have the best luck in the mountain towns with lots of wealthy visitors like Aspen and Vail. Most of the US mountains and northwestern states are too "natural". I travel for work and IMO the best fake boob places in the United States are (in order):
1. Miami /South Florida in general
2. Southern California, specifically the west part of LA County, Orange County and San Diego
3. Dallas
4. Phoenix/Scottsdale
5. Houston
6. Atlanta
7. Orlando/Tampa (northern Florida)
8. Chicago (fake boob capital of the Midwest)
9. New York (probably should be higher due to sheer numbers but pretty confined to select neighborhoods in Manhattan)
10. Washington DC (mainly suburban areas in Virginia and Maryland)