You can buy backer rod at Home Depot. It's grouped together with weatherstripping and similar items. It's the same aisle as water heaters, mailboxes, and for-sale signs. I don't know how those items really go together, but that's the way they do it.
You can get upholstery foam at most fabric stores. If you have a Jo-Ann Fabric near you, then they should have it. There are also some specialty foam stores out there, like
A-1 Foam and Upholstery on Lyndale Avenue. Oh, wait, you're nowhere near Bloomington, so you might have to look for something closer.