I would recommend picking up a map that shows National Forest Lands and unimproved roads. I prefer National Geographic brand, good quality. In almost all sections of interior Colorado you can find an old dirt road somehwere in national forest and free camping.
It might be too far in your case, but I am pretty sure you can find camping up Montezuma road, just turn off at Keystone parking lot and go way up that road, which heads up near the south west slope of Greys and Torreys peak.
Does this condom make me look fat?