It's not so much a special washer as it's a special nut. I don't have a good picture of pne that I can post, but I can give a description. What you need is what the call a trim nut. It's a stamped steel nut, similar to a speed nut, that has a recess in it. That puts the threaded part that attaches the part above the shoulder on the mounting studs. Some will also have a rubber washer bonded to them as well. You should be able to get them at any body supply shop, or maybe the flaps. If I have one loose, I'll take a pic of it and post it.
