I believe you could find a similar foam at Jo-Ann, Michael's, or some other local fabric store. I think it is called urethane foam, but I'm not 100% sure. It should come as something like 4'x8' blocks of varying thicknesses but sometimes the thinner stuff will come on a roll so you can buy a couple feet of it instead of a huge block. It's usually near the quilt batting and pillow/stuffed animal stuffing.
Also, if you do decide to get it from Jo-Ann Fabrics, it is worth signing yourself (or wife/gf if you want
) up for their mailing list. They send out coupons for 50% off any 1 item (a single cut of fabric or foam counts as one item) and often it will stack with sales as well for like 60-75% off that one item. You can always cancel it once you get it or keep it to get things like grommets or nylon webbing and buckles for homemade cargo tie-down straps or grab handles.
As far as cutting it goes, the thinner stuff will cut just fine with a sharp pair of scissors, but thicker blocks or carving it into shape would probably take a good knife or saw. The electric knife idea sounds good, but I haven't tried it myself.