I tried Macs, LMC and Dorman/Help with no success. I idea where to get one of these black rubber plugs? Thanks

 

Try these links:

RUBBER PLUGS : American Mustang Parts, World Greatest Ford Mustang Parts Store

https://www.steelerubber.com/restoration-parts

Might try NPD also.

or Plumbing or electrical supply house.

 

377678-S .. Air Cleaner Plug / Available from Ford.

There are literally dozens of Ford dealers and etc sources in the US that have this puppy, here's some in FL:

Ford dealer in Hialeah has 8 .. Dealer in Tampa has 3 .. Dealer in Royal Palm Beach has 5.

GREEN SALES CO. has 185 and CARPENTER has 220.

The next time you ask a question, please tell us what you're working on, we are not mind readers.

I had to look at more than a dozen air cleaner pics to see what engine sizes used it. There are no parts catalog applications for Standard Parts, what this puppy is.

 

Also every ACE and Fastenal has the plugs.

 

Could probably get away with a 20 cent blind grommet from the hardware/auto shop.