Were can i buy that black elastic material at.just like the one for the shot gun shell stock holder.?

i want to get i thick roll of it if possible.i know how i will make it and all but the question is where do i buy it.it needs to be very wide.and if its in a roll then ill have a long length.i wish i could find something close to it so i can give you guys an example.and i need it to be black.its kinda like the straps on carhartt overalls.so if i can order it or if i can go to a store and buy it please note.