Well ...... actually..... since USFA has gone the way of the Do Do Bird (Darwin award granted) ...... you can't. Unless someone wants to strip out their box to restore your box then having another box that needs foam to restore and looking for someone to strip out their box to restore their box ........ which probably ain't gonna happen.
I have seen USFA Boxes for sale, anywhere from 15 Bucks (know where the term "Bucks" came from??) to 25 Bucks. I do not however, know if those boxes included the "foam." If those for sale boxes include the foam, you could strip one of those boxes to restore your box,
which still leaves you with a box that needs a full foam restoration job, so your hoping, again, for someone to strip out their box to fix your now "extra" box, leaving them hoping for someone to strip out their box to fix their box. I doubt it's gonna happen.
I'd suggest going to Jo Ann Fabrics and Crafts get some block foam and some pleasing fabric and make you own. Really.
Coffinmaker