The only wild herd I've heard of is north of Wellington, it's a fairly new project:
https://cvmbs.source.colostate.edu/northern-colorado-bison-herd-flourishes/. There's not much open prairie in South Park that I'm aware of that isn't ranched, and I've spent quite a bit of time there. The area east of Rocky Mountain Nation Forest, toward to South Platte has some grassland, but it's not nearly as wide open and unpopulated as that between I-25 and 287. Pawnee National Grasslands seems like a candidate, too.
Just as aside, WarBuff, if a buffalo wants to go through or over most any fence made for cattle, it will. There's been a small herd near Wellington Lake (hours from the aforementioned Wellington) for years. I hunt the area and over the years have frequently walked by their pasture. One day the owner asked me to look for the bull - they'd all gone through the fence and wandered down to Chatfield State Park. The owner managed to get the cows in a coral, but the bull was getting pissed and wasn't having it, so they left him. If you haven't seen a mature bull up close, you'd never wonder why Ralphie is a heifer, once you do. So, he drove his girls back the 2 hours home, which is a little more than 20 miles as the crow flies. Less than a week later the big guy was back in the pasture, having jumped the fence to get back in.