I usually buy Salt Cod from either Sessa's in Davis Sq, Courthouse Market in East Cambridge, or the Portuguese market (with Brazilian Butcher) on Broadway near Foss Park in Somerville. One of the downsides when you want a little, is they will not cut the thick end of the fish, but the quality is far superior to what is packaged in supermarkets. A nice alternative is that McKinnon's appears to be regularly carrying Salt Cod, cut from a whole fish, and they offer small pieces of the thicker end. I believe the price was more favorable too.
They also recently have had some nice 2lb packages of roughly 26/31 Tiger Shrimp, for a fairly reasonable price (more expensive than Market basket, but better quality .. prices were also about the same as rest depot and less than defrosted shrimp which most places sell).
They have cut squid rings that look pretty awful, scary battered shrimp, usually some packaged octopus, and occasionally frozen mussels (green lip, etc). In general its worth a peek in their frozen section. I also get them to cut provolone thick to BBQ (marinate it with a bit of oregano and olive oil, just cook it until softened throughout).