I found one original plug on the passenger side. Those found by BobH are a very good match, you just need to gently cut off the "collar" and, if you see fit, shorten them a bot form the collar side with a knife. Then they look like original in my opinion.
Side question: I wonder if those plugs were installed on the outside of the chairs or also on the inside? I would not be surprised if it was only outside - you basically do not see the inner side of the chairs in that place. Always a saving to leave them unplugged...
Anybody found them on the inside of the chairs (in original chairs)?