Here's the FAA list of approved ATP-CTP schools. As far as I know, that's all of them. Looks like Denver area has FTI and International Aviation Services (Aurora, Colorado) as the only two. I have no experience with either. And, you're right regionals are pretty much strictly in-house, if they're even running courses right now at all.
https://www.faa.gov/pilots/training/..._providers.pdf