CJ - thanks for making the interiors section.
MACs in Fargo (and most of the other MACs stores also, I think) carry a pretty complete line of Upholstery materials and supplies for the do-it-yourselfer. They have a good selection of foam. What you really need to know is what type (density) of foam you want. If you are building a seat with no springs and all foam, you want at least half or more of the thickness to be very
firm. Then a layer of softer (but not the softest) for the top layer. I use the pre-backed 1/2" foam for quilting panels, but use a denser 1/2" with denim glued on the back when quilting for seats. The cheaper foam won't keep the quilting (pleats) full as long in a seat and will flatten out in a short time. For a little fuller pleats I sometimes use 1/2 with 1/4 glued together for a total of 3/4, but if you do that, it is tough to do the tuck and roll part...it works good for just pleats with the stitching showing though. Keep in mind when you are quilting - the material and foam shrink with each pleat. Figure about a 1/4" shrinkage for each sewn pleat in 1/2" foam.