Right now there is an entire Apron on ebay but I wouldnt pay that much. Where it differs from other system 11 games is it has metal wires all around the front side of it. The first group of WPC games had those. If you just need the stickers for it, BOP actually has the standard system 11 apron--that is nothing special as far as the name of the game or anything. You can find the entire sticker set for it here at Little Shop of Games: http://littleshopofgames.com/product/system-11-apron-decals/
I purchased that set and just needed the Williams logo. It is pretty close, but not the perfect size if you are applying it over the existing one. I cheated the logo up a little to cover and then the stripes had to be bent slightly down. Mine did come out good however when applied over my old one (that had scratches in the logo), and the lock bar pretty much covers the bottom stripes anyway so it works just fine.
I have the 2 side pieces (lane change) and the shooter sticker still unused if someone needs those. $5 each.