I get it from one of my suppliers, but they only sell to upholstery shops. You don't really need the backing, but I have used generic flat rubber floor mats from Wal-Mart, cut them the shape I wanted and then glued on the carpet and did the binding. Almost any carpet that doesn't have a padding attached to the back will work, but thinner carpet under the nibbed backing works better.
You can get automotive grade carpet by the yard from www.accmats.com
. 80/20 Loop carpet is standard on most older cars. It's hard to figure out how to order cut yardage from them. To do that online, click on "shop online" (You need to know which carpet you want to order ahead of time before you click on shop online.), then click on "select by item" (don't fill in the part about what vehicle you want the carpet for) and then click on "select yardage". If you want softer more pliable carpet, don't order it with the poly backing, that is a stiff plastic backing, although you may like that to make floor mats out of.