johnqhitman,
I had the identical problem. I was unable to buy one at a junk yard or on-line. I could advise you to buy one at the dealer but that is abusively obvious and not helpful to you. Might I suggest jcn37 above described my solution as well as I could. It has worked well for 3 years. The objective is to keep the gas in, keep the dirt out, to not have another unplanned leak. You can cast a silicone seal in a latex finger. Cut off a latex glove finger tip. Fill it with silicone. slip a pen or a tube, the size of the fitting (a little smaller) into the goop. wait a day. A home made cap.