Hi picklepot,
While you certainly can find some foam replacement layers from airbed aftermarket suppliers, I would suggest that you also consider some of the foam suppliers from the component post here. There you’ll have a wider selection of latex, memory foam and polyfoam options, in different levels of quality and firmness. The latex will generally the most breathable foam, with the memory foam generally the least breathable.
Regarding size, some of the foam suppliers will cut to size for you, where others will ship more standard sizes. Cutting foam, should you decide to do so, is quite easy with an electric carving knife, and has been done by quite a few forum members.
If you have more specific questions, let me know and I’ll do my best to answer them.
Phoenix