As far as i know, OEM seat foam blocks are not available.
Try an upholstery shop, they should have firm urethane foam. Most upholstery foam is too soft but what you want should be available - they may have some firmer stuff used for ATV/snowmobile seats. For a really nice approximation of the original seat you should use a thick layer of quite firm foam and a top layer around 3/4" thick of somewhat softer foam or even "memory" foam.
Standard upholstery foam is rated by pounds per cubic foot or something similar, the weight of the a volume. Heavier weight = more plastic = firmer. Sorry but I have not done this in many years and don't remember the grades that are best.
Definitely try sitting on a block roughly the size of the seat. Too-soft foam is not good, you will compress it completely so you feel like you're sitting on solid metal. Your butt should only sink in an inch or two. Note that the foam will get less firm with use so starting out a bit too firm is a good idea.
You will have to carve the new foam block to fit, you can use the old foam as a pattern if it isn't completely gone to dust. A very sharp knife cuts it easily, a new Olfa box cutter blade for example. It does not need to be perfect: the original block was moulded rather than cut to fit, but the elastic nature of the foam lets it fill the seat cover OK even if it looks a bit rough. You can glue the block to the pan with some contact cement to make it stay in place while you fit the skin.
