It depends on your intentions of subspecies, from the sounds of it, you're going after a Merriam. If you want a "pure" Merriam, NW Nebraksa, in the farthest portion is definitely a good place to go. South Dakota would be the next suggestion as far as much accessible land to choose from. If it's not a particular bird your going after, and rather just a traveling hunt, I'd recommend NW Kansas. There's dozens upon dozens of WIHA properties and plenty of WMA and state lands to access. This region will have primarily Rio hybrids, but the plethora of birds should make for a very enjoyable trip. Truth is in Nebraska bird numbers have fallen, and in my opinio, there's no better place in the country to public land hunt right now than Kansas.
I'm a Kansas resident, and grew up in Missouri, so I get to hunt MO/KS very hard and could tell you about spots across the whole portion of both of those states. I'm gaining knowledge on Nebraska and Iowa after the last couple seasons, and looking to make them both annual spots. PM me if you need any more information, or have any other questions that I may be able to assist you in.