IMS Museum has several and most of the Champ Cars up to the mid 60s were dirt cars as well. They have the Grant King Foyt car there you see on all the Terre Haute flyers, posters and paintings. with just one wheel on the ground.
https://www.google.com/search?q=AJ+F...tm%3B737%3B598
Not sure if they'll be open but you can see the majoity of race cars they have (About ten) Through the front window At the 500 Wheels Museum in Terre Haute on Ohio Ave.
Many cars are in private collections and only open either never or on certain dates. Im sure others have some info on where you can see some in open places. Someone said theres a good museum in Kokomo. I haven't stopped there yet.