Foam is expensive, plan to do this once, use quality foam that won't break down. Its a pain to do it yourself, I would have it made to order.
some rv people have used:
https://www.thefoamfactory.com/https://www.cushionsource.com/catalog/rv-cushionsHere is some reading to see what others have experienced:
these are older threads so the prices are off.
Its said to not use more than XL 38 foam otherwise its too hard a foam.
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/23411753/print/true.cfmhttps://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27777972/print/true.cfm