Here are two pictures of the spring nut in question, the first is from the AMK Guide to Ford Fasteners (page 212) for the style, the second was the closest I've been able to find and modify to look correct. Locally, we have one of the better hardware stores around (Orchard Supply, they're willing to go the extra effort) and even their searches have not turned up anything close.
One thing did pop up in my latest go-around with the Tinnerman catalog and I intend to go back to it, the acorn nut, 374073-S8, used on the center lower grill cover on 66 Mustangs. It's shown in the Ford Car Parts ILL 82 page 3.
Jim