Hey there, welcome to the board.
I have never personally looked for a frame before, but I have two suggestions.
1. There is a place in Cambridge MN called Trucks and Toys, they are always advertising old cars, trucks, and projects in the Tradin Times, Auto Trader, and Truck Trader. Might want to look them up and give them a call. If they say "no", then ask them if they have any leads.
2. E-bay. I see alot of people parting out 73-87 trucks on there. Look in the "parts" section, and also in the "parts car" section...I have seen bare rolling frames from time to time in there.
BYW - Where in Rogers do you live? I live in Coon Rapids off of Foley Blvd and Northdale Blvd...near hwy 10 and Hanson.