Where can I find the drain plugs for the outer wheel house drain holes in my 1968 Camaro?

Im guessing you mean the small round black plastic plugs that are in the rockers and wheelhouse
They put these plugs in just about all GM cars and trucks. Just take a screw drive and find a parking lot and load up! LOL. If not just go to a junk yard and pop a few off a GM Car or truck.

Actually they are rubber on the first gens. I think most of the repo houses carry them.

The one I removed were plastic. I searched YearOne, Rick's, Classic, The Paddock and NPD and could not find any part number. I'd like to buy new ones if possible.

The plugs are a standard sheetmetal item. Available at most better equipped parts houses. Dorman, Auveco, and others, make them.