Felslate is the rare spawn of Leystone viens. So the maps are the same as Leystone Ore maps.
However, Felslate can also be farmed by slaying and mining mobs like Felfire Basilisks in Suramar. It is a reliable source of Felslate, but it will be packed with other players.
Happy farming and hoped this guide helped with farming Felslate Ore in WoW!