Where can I get the "U" nut on the bottom of the fenders?

My car is missing two (1 on each side) and I was wondering if anyone knew where I could get a couple? I wasn't happy with the panel nuts they had at the local hardware store. I thought I could do better.

These are the U shaped panel nuts that hold the 5/16-18stud on the bottom of the fender behind the front tire.

Any help is greatly appreciated!