Mine were just an all-thread about 2 3/4 or 3 inches in length with a common thread size. You can find something that will work at Home Depot. If you don't have one to use as a sample, I can pull one and give you the specifics.
The nuts that were used with mine were only about 1/8 inch larger than the hole in the slate, so I added a 1 1/2" fender washer to give more purchase area on the slate.