Mineragua by Jarritos is comparable. here’s a page on the water test. But if you can’t find Topo Chico in SoCal I’m really worried. Find a local Carneceria/Mexican grocery store and they’ll have it in abundance in my experience. I live on CA too, they generally always have some in stock.
Morton’s Salt Substitute (KCl) plus LaCroix plain seltzer
Sold at Safeway, Nob Hill/Raleys, BevMo, and Lucky (NorCal)