Good luck in Iowa! You might find some pawpaws. Look for woods and creeks, especially where forests border fields/clearings...look along this interface.
That's about all I can offer.
If you make it to Nashville any time, please let me know! I'd love to go Pawpaw and Maypop hunting with you.
EDIT: I would add that you may not find any ripe pawpaws in mid-September. Especially being a cooler zone, they may ripen even later in Iowa if you can find them.