Not much to buy in terms of souvenirs here, but there is a nihon-shu sake that is made in Otsuchi that is delicious. Just go in any liquor store and ask for Otsuchi no nihon-shu, and they should be able to help you find it.
Also, Mast has anything you could possibly need. Grocery store, a Homac, a department store, video store, shoe store, a cute little hippy/Indian store.