The Bullitt Mustangs and Audi TTs have nice looking units that are readily available at the parts counter or Ebay. I've heard that most motorcycle ones aren't vented so you need to make provisions to vent the tank or you might have problems.
or the real oem deal for about $200-250. Just copy and paste the following into the eBay search field to find all the matching items.(1971,71,1972,72,1973,73)+mustang gas cap mach*