In Colorado and Utah there are numerous dispersed camping areas run by the Forest Service and BLM. These are areas without designated campsites, but have a designated area for camping. Some of the nicer ones have a vault toilet or Port-a-Potty, and a water spigot (seasonal) somewhere in the area. I have never heard of any of these having reservations, just first come-first served until the limit is reached.
On-line research will bring up most of these.
camping Colorado and adjacent states one weekend at a time