As you run, ride or fly South from Little Ala Mhigo in Southern Thanalan you enter Zanr’ak. The first Tortoises you see are Bronze ones (around level 43s). You need to head deeper in until you reach Iron Tortoises, which are around Level 46 – and slightly larger than the bronze ones too. They do roam about a bit and seem to often be on their own – except in the FATE below, anyway.
Involved in FATE
While scouting for landscape Iron Tortoises I stumbled on this FATE. It seems there will be plenty of them for you here.