There are a great variety of shops in Fronteras (the town by the bridge). The one I like best is Tienda Reeds, also known as Chiqui's, the owner's nickname. He is well stocked, including supplies for boat. He is located just upriver of the bridge, on your port side while going upriver.

Another choice is the Bodega Familiar, close to Bruno's marin, which is an inexpensive supermarket belonging to the Paiz chain, now partners with Walmart.