Its a pretty common Plastic Pop Rivet, just like the metal pop rivets only bigger to make up for the lack of strength and much easier to cut out with just a knife instead of having to drill them out like the metal pop rivets. They are NOT often found at the Auto Stores, BUT I have found them at bigger hardware stores that have a big section of fasteners. I found them online to order pretty easy, just google Plastic Pop Rivets, you'll get several types including the plastic push fasteners come up, but in the list there will be the exact type you need.
They fasten with a special plastic pop rivet gun, BUT, it works the same way as the metal pop rivet gun. I have a cheap metal pop rivet gun, it didn't have a fitting for stems as big as the plastic rivets, but I was able to improvise to get it to work and it does just fine. i.e. I removed any adapter off the snout and inserted the big plastic stem into the jaws without any adapters or fittings on the snout, and used the tool just like a metal pop rivet. It worked fine except more of the stem was left sticking out of the rivet then should be, since its plastic you can easily cut it off. Without the adapter on the tool, the remaining portion of the stem won't simply fall out of the tool, (the adapter opens the jaws holding it when you release the handle) but with a small screw driver I was able to pry the jaws open and get the remaining part of the stem out of the tool. NOT ideal, but if you only need to do a dozen or less, it will suffice.
I believe the size was 1/4", BUT, its been a while since I've done the job, so confirm the size you need. The parts catalogue will specify the size in it, so look it up. Or at least measure the smallest hole of the parts you'll be joining and use the size of that hole as the size plastic pop rivet you'll need.
Last edited by Mongo; 12-17-2015 at 11:00 AM.