Maybe a shop that does undercoating ?--I used to sell bags of those pop in plugs to body shops ,they used them to cork up holes made for applying undercoating on door jambs.etc...
The brand we carried was W&E,they made just about every type of body moulding clip,plugs,etc..a body shop supply store will likely have some..(or you could visit a pick-n-pull and pop some off junk cars ?)..
That plug will need to be pretty big,it looks like its 1" ?..ones I sold were only like 1/2" or 5/8"--maybe one of those rubber expansion plugs to replace freeze plugs they sell will work for you ?.