The foam that I use is 4 inches thick before it has the factory skin removed, leaving just under 4 in. (3 3/4 in). It is referred to as Y20 and is difficult to find in small pieces - usually around $10-15 sq.ft. It comes in a 4 by 6 foot sheet which is much more than you will need and more than you need to spend. Unfortunaltely one can only by it this size wholesale if you are a company. Since it so expensive, I usually make the seat for people at a low cost for labour. That way, if I make a mistake and have to start over, the customer does not have to pay. Various custom kayak manufacturers have the foam. Don't be mistaken for a lesser quality grey foam used for roof mounting blocks for canoes and kayaks - it is not as dense as the Y20. Depending on where you reside, I can supply you with a the foam you require.
Rod