Have you checked The Jaconson Garden in Marshalltown?
Location: 2554 Dillon Rd. Marshalltown Iowa Phone (641)479-2632. They have 400-500 varieties
Also There's a place I'd love to go to called: Pantheon Hosta Gardens in Dallas Center Iowa. They boast 1100 varieties.1939 T Ave in Dallas Center,Phone (515)992-3805
It sounds like a beautiful place to visit.
Go to Google and search:AHS Garden Directory Iowa
You have to have Adobe. There is a full discription of the Hosta farms in Iowa. Good Luck, Sandy

It's also available as a web page.

Here is a link that might be useful: American Hosta Society Garden Directory Iowa

According to my wife, who grows and sells hostas near Ames(high-quality ones for very reasonable prices if I were to give her a plug), Car/Mel gardens is in Altoona and is operated by Melvin Visser. I can dig up the contact info if necessary.