First decide if you want what NWTF says are the different subspecies or if you want to kill birds in their native range. NWTF recognizes mongrel birds. You can do the slam with greater ease if you agree with NWTF.
Native, unquestionably pure strain birds limits you to the Southern half of peninsular FL for Osceola, TX and OK for Rio, and NM or CO for Merriam's. You want to be in the mountains of NM and CO to avoid Rio mutts. There are Easterns in TX and OK, too, but avoiding them is easy.
FL birds are the hardest on public and most expensive on private. Start there and it's all downhill difficulty-wise. The FL bird's reputation for difficulty comes from the terrain and pressure. You can kill one on public, but most people are money ahead by just paying to play on unpressured private birds to get it done in one go instead of having to go back. Back when I lived down there I'd see plenty of out of state tags in the fall trying to finish a single season slam after they blanked in the Spring. Those extra trips cost.