there are thousands of stores so how do you toggle? Pure chance?
Pretty much. Target lines up the stores by zip code, so I just try one after the next. The search is sporadic, not organized. I'm spending way more time than I should on this.
Sometimes if an item in a certain category works for a store, there is a chance that other items within that category will work as well. Once I found one kosher food item working at Gateway, the rest worked like a charm.
Items that say "price not available" and refuse to add to cart in one store, might very well add to cart when you choose another store.