Yeah, not many used tool places in the denver area. I have no clue what might be available in Pueblo or Colorado springs.
There is only one Flea market:
https://milehighfleamarket.com/en/
Here is the website for Charlie's 2nd Hand Store. They have been in business since 1928! Current third generation owner Steve is a great guy, just tell him what you are looking for and he'll help you find something! The differing name on the web address can be confusing:
https://www.usedtoolsplus.com/
This used tool store is fun too, been around for 70yrs. They have more hardware than "tools", but I always find something to buy there. Over the years, I've seen a barrel of tommy bars and buckets of bonney wrenches, all the same size and all rusted to hell (from the flood back in the 1960s, ask them about it, crazy story):
https://www.surplustoolsdenver.com/
FYI, don't wear nice clothes to the tool stores. LOL and bring something to clean your hands. I always get grubby hands when I go!