For future references, the part number is 1S3-21875-00-00 . Description is just "cap" . The price is $2.97. This is an item, I'd hassle the local stealership to order, b/c now days, nothing is ever kept in stock. Everything is ordered via web. However, pending location, the shipping cost exceeds that of the actual item. So, that being said, let the Big Money Stealerships pay shipping!
Also, they come off from back pressure, and not being fully on to begin with. I bet money, 90% who have lost one, lost it after they had recently "put it back on". You have to take the time to be sure, the rubber is completely flush, all the way around the plastic lip!