We got ours at OSH, a home improvement warehouse. A black plastic plug, in the hardware section. Home Depot out here didn't have what we were looking for. You might want to try other home improvement stores in your area as well.
I just went to some mom and pop hardware store and bought a plastic plug that almost matched the color of my car and then just put some exterior waterproof silicone around it to get a good seal and that did the trick just fine!! No leaks or problems!!! Good Luck with your project!