Contact the ranchers associations in the area you want to hunt and ask them for suggestions, also the states DNR may help.
There are some PD forums/web sites but I haven't had much luck with them, they seem to lead you to heavily shot area's.
Medora ND was a good place for us years back I had contacted the RA out there and she gave me probably a dozen ranches to shoot on and who to contact.
Also the Eagle Butte area was good but a bit more heavily hunted, least it was years back.
We got a few spots there by asking a local policeman.
Watch out for places like Rosebud, not only do they want a licence to hunt, but last I heard they still want a guide to go with ya.
No Guide, no hunt.
I was hoping to get out one last time before I sold my varmint rifle but don't look like it..if there's dogs, it's a lot of fun.
Geysers of guts...lol
Remember our vets, they need our help, just like they helped us.