This is a bit out of the way, but I went to a crab-focused service station （道の駅）on the Sea of Japan side. Huge crabs for very cheap price. For miso soup, they literally cut a crab in half and put half the crab in the soup, bringing it to a hot boil. We got 2 large snow crabs (ズワイガニ) for ¥2200. We actually only paid for one -- the shop gave us one on the house. It's a whole courtyard of crab shops (and other seafood shops) where you select and buy. They provide a dining area to eat afterwards. Besides crab, they have other seafood like grilled shellfish as well as sushi (we tried a platter of maguro for ¥1500 - 2 akami, 2 chu-toro, and 2 oo-toro).

Like I said, it's out of the way (it's between Toyama and Niigata) and probably only accessible by car, but well worth it if you're in the area. You can also add in a stop to the very nearby island shrine (Benten Rock).

Oh the place is called Kaniya Yokocho Marine Dream Nou (the name of the city / village is Nou).