You can get quite cheap, plastic things from the american army surplus - not sure if they're what your after though.
Something like this - https://www.ebay.co.uk/itm/MOLLE-Gen-IV-Pack-Frame-for-Large-Rucksack-New-Genuine-US-Military/152405368223?hash=item237c115d9f:gWMAAOSwc~5a~Yek
I'm not sure exactly what you want but I bought a Swedish LK35 metal frame rucksack for ?20 (+postage). That was in A condition, A+ condition was a bit more, B condition (needing a few repairs) was only ?15. militarymart.co.uk