Anywhere that sabertooth salmon spawn, the otters have a chance to. Best option is to kill all the fish, barys, spinos, hesperornis, everything around a pond, wait for respawns and check for otters.
Double pond near Green ob is the best place I've found - I've tamed 10+ over there
Or, if you're just bored of looking, and you're Xbox PvE New, I've got both non-mut and mut otters for trade