Re: where to purchade seat foam?
See if you can find a local upholstery supply shop, a place that supplies upholstery shops. I picked up a few slabs of foam from the guys below and thought they were very reasonable. The hard part was determining which foam I wanted as they had about 20 different densities and 2 qualities, economy grade is (was) 1/2 the cost of their high quality grade.
Their website (or lack there of) is pretty bad, but it's a mom-n-pop type place. From what I remember they number their foam 1-20 with 20 being pretty hard. For my boat I chose one of their high density foams (guessing #17) for the 3" thick butt section and medium/high (guessing #12) for the 3" thick backs, it turned out great, very comfortable. The nice thing about this place is that they can custom cut your pieces, or sections big enough so you don't have a lot of waste cutting it down. They have huge blocks of foam in their warehouse and cut it to order.
http://www.galaxyfoam.com/portal/