The little plastic retainer on for my gas cap snapped off. The leash for the cap is still fine, but the push-in retainer snapped off.

The retain resembles the retainers used in the door paneling, but it's smaller.

Where do I go to find a new plastic gas cap retainer?

 

rarrr, go to your local auto parts store and look in the fastener section. You should find a bunch of the "friction clips" (that is what they are called) and you can find the correct size to fit on your car. they are just a few bucks and will probably be labeled for another brand vehicle. Odds are, you will have the best luck looking at the ones made for Ford.

 

I had the exact same problem on my 2003 X-Type. AutoZone had black retainer pins that fit and I think I got 10 of them for under $3. Easy quick fix.

 

At Autozone I bought the Dorman 15/64" Nylon Rivets part number 961-040. This worked perfect.