Do they have a clothing store where you can buy shirts, caps deitcated to going to The Hocking Hills?

There is a gift shop at the park campground office/visitor center. They have a wide variety of affordable shirts. There is also Grandma Fayes which offers a huge amount of sweatshirts and souvenirs just up the road if you keep driving past the road to Whispering Cave and the cabins. https://www.facebook.com/pages/Grandma-Fayes-Grocery/151519008193617

There is a souvenir shop at the main gate where you go in for information about the park.