Re: Upholstery foam..
I used 4" high density foam from JoAnn fabrics. I don't know how the price compares with online foam, but it's certainly convenient. For cushions, do not buy anything less than 4", you will bottom out. I strongly recommend that you do not buy closed cell foam. No one has recommended it on this thread, but some have in the past. IMO, it is very uncomfortable and suited to outdoor applications like cockpit cushions.
Sabre 38 "Victoria"