You talking about the round hole in the bumper or the square plug that fits in the receiver?
I had a plug for the receiver that plugged into the trailer pig tail and was a 4th brake light ripped off in the Wally World parking lot about 10 years ago while in Florida visiting my parents and a chrome Bully step off of the receiver of my ex's Grand Cherokee right from my campsite while I was camping and trout fishing about 8 years ago. THAT ONE REALLY PISSED ME OFF because I had a hard shell topper on top and had that step on there to be able to get things out and put things in it!
Wasn't long after replacing that particular item that I invested in a couple of locking hitch pins I still use to this day!!!
Anyway, if it's the square hitch plug for a receiver, any auto parts store, a plain jane plastic one with the 2" square plug and the tube with the hole for the hitch pin is about $5.
If it's the round hole on the bumper, Upperbound1.com has a rubber plug for that hole for $3.95 with free ground shipping...