I have to admit that when I first read the title I was wondering who had leaked my secret, so it was a big relief when I read the rest of the post.
Lowes' Bldg. Supply and Home Depot have a ton of assorted fasteners that can be used in autos. I think what you are looking for are called 'push nuts'. Take that one and run with it! Seth
FYI - Ace Hardware blows Lowes and Home Depot away as far as their selection of fasteners and related items ...