I know there is a big Joann's on S Colorado Blvd in Denver just north of I-25. I don't as yet know of any LQS in Denver.
My daughter just moved there and decided she wanted to get into quilting since she has a friend expecting a baby. I checked out the Joann's when I helped her move to Denver (job transfer) but didn't find anything when I did some quick Internet searches. I did find the addresses for 2 sewing machine repair shops, however.
Also, check out the Goodwill off of Broadway in Denver - It's huge and really worth taking some time to visit.
Good luck. If you find anything, PLEASE let us know!
P.S. I don't know Boulder at all. Hopefully someone does on the board!