For mobile:

On single player, search the bottom of the ocean. Tusos tend to spawn in open areas and one of the best spots on mobile is the oil field off the coast of herb island. Breed or tame high health tanks like mosas or basilos to tame. Recommend using 10x or 15x balms.

For servers, tusos can be even rarer because of the high demand for them. Search the entire ocean, surface and depths as creatures on servers tend to move around constantly (have seen and heard of tusos than come right up to shore). High health tanks recommended again and usage of balms highly recommended