The originals are called molley nuts, or jack nuts, or molleyjack nuts, depending on who you ask. They are available from Ace or TrueValue hardware, but they generally have to be ordered, rather than found on the shelves. I believe they are 10/24 X 5/8". Doug Kinney at fierowarehouse.com
should have some in stock.
You can also use the rubber jacketed brass insert mushroom nuts which can usually be found at your local auto paint supply house. Might check with them on the molley nuts too. They use the same system to mount some outside mirrors, but the size is off. Closest I've ever found the rubber mushroom nuts was 10/32 X 1/2", but they still worked, you will just have to get new screws to match. Could do a search on the net for "Hillman" which is who makes the molley nuts. Good luck!