I like ravines to look for ore berries, most ravines I've come across in Unleashed have 1 type of ore berry or more. Iron isn't the most common though, but nowhere near as rare as gold or silver (or at least that's my experience with finding ore berries so far).
There are also trades with villagers to get ore berry bushes, so it might also be worth to look up a village and check if the village Tinkerer sells them (I think it's a guy in a dark blue robe, who belongs to the TiC house in a village). They want weird stuff though, I think pistons or something like that, so check what they want carefully before running over there with a ton of Emeralds