Common Substitutes are other types of flat rocks ( I'm using red shale), inverted Clay pots, PVC Pipe ( I recommend 2" diameter or larger and the piece should be as tall as the tank), Large or broad leafed plants such as Sword Plants ( live or fake), the acrylic breeding slate or the Clay breeding cone. As long as the fish have space to lay copious amounts of eggs on them, all should work.
Hope this helps