I looked under "Maverick parts" at Melvins and Autocrafters and didnt see them. I will try looking under Mustang parts. I was`nt sure if they were the same size on Mustangs.
Melvin's has 7/8", 1", 1-1/4" and 2-1/4" rubber plugs on their website. Also got the funky looking drain plug that was used on the 70-71. Click on "RUBBER PLUGS/GROMMETS" on the menu. http://www.parts123.com/parts123/yb.dll?parta~partsort~1~cadffejo~~20