Colorado has leftover licenses available on the CDoW website, updated every hour or so to be the latest info.
However, hunting on the plains here is almost ALWAYS on private land, and licenses go by WMA areas overlaid on the land. The website has a handy map of all the WMA's, but it'll be labor intensive to track down individual landowners to get hunting pemission.
Colorado has a useful Big Game Walk In program that costs hunters $40 and allows you to hunt, without having to seek permission, on private ranch land. So co-ordinate the leftover tag you want, by WMA with where the Big Game Walk In program has a holding- and you are hunting.