Fastners for cement shingles to sheet rock

[tpf903]

Have a 50's house. The outside sheeting is black sheet rock. There are asbestos/cement shingles attached with special fastners.
I was told that they are stainless steel "nail" that resembles a cotter pin. When hammered thru the shingle into the sheet rock, the loop gets mashed and opens up the split end, holding the shingle to the sheet rock. I was told they were called ES Nails.
Where can I get some?