If you are looking for a school for Fitness Instructors. Check out NPTI (National Personal Trainer Institute) in Lakewood, CO. Highly accredited.
If you are looking for a location to take the cert exam, check with the organization you want to cert with, like ACE, ACSM, etc. There are a few testing centers in Denver.
AFAA holds APEX in area twice a year for various certifications at a discount.
In addition to NPTI, Arapahoe Community College has a 2 year AAS degree program that is well structured and gives you a lot of information to prepare for the ACSM CPT test. However, you don’t need a degree to get that. It does help when you’re looking for a job – something that makes you stand out from the crowd.