Where can i find a large roll of plastic i.e acetate/polyester that i can use to make street art stencils?

You can buy a large sheet of thick polythene from most builders merchants. It's used as a damp-proof coarse under a concrete floor, ( before it's poured that is). You don't necessarily have to use Acetate, that's being a bit of a purist. Using polythene, you might find that you also need to use a few double sided pads to hold the image close to the wall, (Pritt Buddies), whilst spraying.