Actualy, Cozumel is on the Caribbean side, not the Pacific side.
I am assuming you mean the Mexican Riviera which includes Cabo, Mazatlan, Alcapulco, and Puerto Vallarta (PV). While I can't speak for the other ports, you can find a gazillion silver shops in PV. Most will bargain to some extent. You should listen to the shopping talks aboard ship as well.
There are a couple of shops we've had some experience with. They are located in the older part of PV, across the Cuale River on a street called Olas Altas. The shops are opposite the Kiaser Maximillion (sp?) restaurant.
On the opposite side of the Malecon, the mile long seaside promenade, you will find several shops selling silver as well. Make sure it's stamped .925 to ensure it's sterling. If you are looking for fire opals, the best bet are the 2 shops on Olas Altas.