What works surprisingly well is to open maps.google.com and search for "naturist resort in Indiana". There are actually quite a few resorts in Indiana.

We were travelling through Indiana a couple weeks ago and spend a Friday evening at Sun Aura resort. If you've not heard of them, they have the largest leg sundial in the world. The accommodations were very basic cabins with no bathrooms, though near the community bathrooms. It's adult only and more of a party place. Ponderosa Sun club is just about right across the street. Next time we're passing through we're going to try it. It's much more the regular AANR type resort.