I created this county heat map using our 90" List. It shows every county where a 90" pronghorn has been killed. The most 90" bucks come from the greenest counties while the fewest come from the red counties. If a county is still white then there haven't been any 90" bucks killed there that I know of.
If you're applying out of state and wanting to hunt the biggest buck possible, you should be applying only for the counties that are marked in the image. And the greener they are, the better. You won't necessarily be able to hunt a 90" pronghorn by applying for these counties, but you will be assuring yourself that you're hunting in an area that has the best genetics. These areas will generally hold more 82" bucks as well. The red counties can be tricky as most of them only have one 90" buck to their credit. Avoid them if you can, but if you run out of options they're usually better than the white counties.
As you can see from the map, in Arizona, New Mexico and Wyoming a 90" buck can come from almost anywhere in the state, but the greener areas tend to produce them at a much higher rate. States like Utah, California, Oregon and Colorado have only a couple areas where the genetics have ever been good enough to produce a 90" buck. Keep this map in mind when you're applying for your tags - hopefully you'll draw an area with a history of great genetics.