Singin, it looks like there's nobody out there to answer your question regarding Merriams guides in S.D. I'm sorry I can't help you out with your question either, really, but I thought I would at least give you some ideas on which way to go with your dilemma.
First thing you should do is contact the S.D. Game and Fish Department and see what info they can provide you with. I believe there are some Indian Reservations in South Dakota that supposedly have a lot of turkeys and offer reasonably priced hunts, as well.
Unless you are just determined to hire a guide to take you, I would also look into putting together a do-it-yourself hunt on public land somewhere. There are a lot of places that have abundant Merriams gobblers on public lands throughout the west. The idea that a hunter cannot find quality turkey hunting without hiring a guide to get him on private land most certainly does not apply for hunting Merriams gobblers in the western U.S.