See them most often in the east swamp on the Island. Suspect it’s because of the amount of creatures that get eaten by the megaladons or wander off into the ocean. They have horrible survival rate due to the hostility of the swamp. Try masacring the dangerous creatures to refresh their respawn and secure the area for them.
Dimetrodons have a tendency to clip through the swamp floor with only their sail sticking out. This makes spotting them considerably harder, best spot to hunt is the fardest eastern part of the swamp. Just fly around the edges or through the central river. Argy recomended, they dont seem to agro onto you when you pick them up making transport easy.