Fjordur has one cave where Magmasaur eggs and nests can be found. The entrance to this cave is found at the bottom of the Wyvern trench at 96.2 Lat 79.3 Lon. Travel the length of the cave, and there you find Magmasaur nests, hopefully with eggs. The approximate coordinates of the nests are 90.6 94.7.
Good luck getting Magmasaur eggs on Fjordur, they are well guarded, and the heat is extreme, so prepare for both of those hostile elements.