Mattress foam

I have a lightly used queen mattress that will be the main mattress in Charley. The loft is a different story, its a 24 inch deep by 58 inch wide area for the dogs or small child. So I am thinking just a simple foam mattress that would do it.
WHere would one start to look for foam, not sure a Michaels or another craft store would have it.

WHere would one start to look for foam, not sure a Michaels or another craft store would have it.