I'd suggest you ask other cruisers who are there, but many of them choose to head
up to the Gulf of California
for the hottest summer months. One who hasn't done that, though, is CF member
zeehag. She's down in Barra de Navidad, but she would be a good source to check with. Even if she doesn't know specifically where you can get new flares, she'll be likely to know who to ask to find out where to get them.
Among those who have headed up to the Gulf for the summer are Rebel Heart (Eric and Charlotte) and Bumfuzzle (Pat and Ali). You can reach them through their Rebel Heart and Bumfuzzle blogs, respectively. They have a LOT of info on the PV scene and could likely guide you to the best local source for the flares (if one exists).