C-Forum

Found this site,Their prices seem pretty good.
They also carry neoprene.
http://www.foambymail.com/Minicel.html

It's nice to have options and these guys seem to have a wider variety of thicknesses, but...

http://sweetcomposites.com/Minicel.html appears to have much better prices.

try kayakoutfitting.com. its about 8 bucks a sq ft for three inch. they also have all different sizes, even 4 inch thick foam. neoprene as well an d check out the skinz stuff its awesome under your feet! real tough stuff.
tim from ohio