Herbs in the Mercado or another store...here you go

There is a store on Benito Juarez (right side heading downtown) just before Ejido street that sells a large variety of herbs. If they don't have salvia I would try asking at one of the stalls in center of the mercado. If they don't have it they might know where you can find it. They serve a pasta with salvia at La Terracita, so I would think that it is available somewhere in Zihuatanejo.