The Game and Fish department, and I think the nearest is in Gillette for Thunder Basis, can provide you with a list of landowners who will let you hunt their land, perhaps for an access fee. Some access fees cost more than others, so call around. I'm making my first hunt this year and rather than chance trying to hunt on public land where there might be too many people I arranged to pay $100 to hunt for two days on a small 1200 acre ranch/farm.
When I'm out in Wyoming (I'm in Texas) I'm going to look over the Thunder Basin National Grassland and ask around about what hunting this area is like. I would prefer to hunt in wide open spaces such as Thunder Basin, but I don't want to share it with 5000 other hunters. I'm not particularly sensitive to the size of the horns of the animals I hunt. I just want a quality hunt: the opportunity to execute a classic stalk and cleanly kill a pronghorn which will be good to eat.