Your local post office USPS should carry flat rate boxes that are free to get.
If you don't see them in the lobby you may have to ask the postmaster for some flat rate shipping boxes.
If they don't have any USPS flat rate shipping boxes in stock your local post office should be able to order you some free boxes.
You can also order some free flat rate USPS shipping boxes online as well and they'll ship them to your address or your local post office free of charge.