Without a pair of calipers it's a little awkward to figure out exactly how thick it is. The rim looks _about_ 1/8th of an inch thick, but i can't tell if the rim part is thicker than the body or not. So it's probably 2 or 3 mm (i think they only do neoprene in whole mm increments?) I'm also not sure if that's all neoprene or it there's also some kind of cloth layer. shelleycat's friends said neoprene has different textures on the two sides, but i dunno, this seems really soft and fuzzy for rubber...
I'm also really unsure about what the durometer rating is. It's definitely soft, but i'm not sure how soft. Is it closer to a rubber band, or tire rubber? Or a bicycle gel seat? (Having never used a bicycle gel seat that last is an especially poor comparison point =P)
This place has lots of neoprene sheets in different thicknesses and colors, but they don't say what hardness. It also costs about $50 for a 4' by 3.5' sheet. There's a lot of different listing of neoprene on ebay, but all the ones i've looked at so far have been of 60 durometer and the listing of uses seem to be focused on construction type stuff.
I'm thinking of finding someplace that sells used wetsuits and seeing what kind of neoprene they use. Not only would that give me a better idea of what it is i need to get, but it might end up being a lot cheaper than buying new sheets that way =P