The best foam is designated upholstery foam and is sold by weight per
cubic foot. Upholstery grade usualy starts at about 8lb./cu.ft., the
'heavier' being the more preferred. Even cheaper grades of such foam
can be made to support more weight by glueing (contact cement) a
polypropylene felt (scrim) on the top surface of the foam.

Most large cities have 'fabric districts' that also include foam
distributors .... who will usually precision cut it for you (in a
special band-saw) for a nominal extra charge. You will soon earn that
upholstery grade foam is quite expensive. When you go to these
specialty suppliers look like a tradesman, make up a fake business card
on your computer -- "boat canvass" etc. tell them you are just
'getting started' and have a smalll rush job for a client, and then
politely 'negotiate' for their 'to the trade prices'.