The stock I use for grafting is either pereskiopsis spathulata or selenicereus grandiflorus. These I had to order them outside of Greece. The only cactus you can find in the wild here is opuntia ficus indica, and after many experiments I came to the conclusion that is very hard to graft. I have only succedded with very few scions. The only that is easily take is  pereskiopsis spathulata: