A New Cage

Perhaps try lino? As long as you make it secure enough so they can't chew it, it will make a nice waterproof base and you could put it over the wood to make it a little sturdier if you wanted.

Have you tried the 'I found mine' function on the main part of the site in looking for coroplast? If you've exhausted all the other options on this page https://guineapigcages.com/cubes.htm then you may find that someone has listed a source close to you on there.