auto parts stores don't have it, and if they did they would charge you out the ass. Go to a sign shop, you know a place that make banners, signs, auto graphics (yellow pages). Just ask them how much they will charge you per sq foot for vinyl. They should give you a good price because you not having anything made, just getting a sheet of the stuff.
once you have there price figure out how much you need, lets say 36"x36" to start. Multiply 36 x 36 you get 1296, divide 1296 by 144 (the magic number), you get 9. 9 times there price per square foot will give you your total.