Eat Fresh, Eat Local.  Cape Cods Fish Markets 

A simple Google search of "Cape Cod fish markets" brings up a map that shows that every town on Cape Cod has a local fish market.  Some are right in the harbor and all of them are less than a few miles from the pier.  Some of them own the boats and some of them work with local fishermen to offer the local catch. A few are attached right to a restaurant ( Pictured above: Fishermans View Sandwich Marina )  No matter where you are on the Cape you are guaranteed to have fresh fish at your local market.  

Here are some of the ones I know and recommend. 

The Lobster Trap ( Bourne )  Restaurant, Fish Market and Seafood distributor

Fishermans View ( Sandwich ) Right on the water. Market and restaurant.  Sushi chef!

Superior Lobster and Seafood ( Sandwich Marina ) Retail market

The Clam Man Falmouth 

Mac's Market and restaurant

Chatham Fish Pier and Market 

Petersons Fresh Market 

Osterville Fish Market 

LIST and REVIEWS FOR CAPE COD FISH MARKETS - YELP

From Pier to Plate  - Cape Cod Chamber of Commerce List