There's a mental process of figuring out which stores are likely to sell a given product, but the methods that worked in the U.S. don't work here. For example, in Boston a corner store was likely to sell basic "bread & butter" products, but if you wanted something interesting - multiple varieties of salsa, farmers cheese, macaroni & cheese with pesto sauce - a big supermarket was a much better bet. Similarly, to buy some nails a local hardware store was fine, but if you wanted options, you'd be better off going to Home Depot.
Here it often seems to be the other way around. We went to a massive Jumbo supermarket today, and they had an entire aisle of oils - thousands of bottles of vegetable oil, olive oil, more olive oil - but no sesame oil. The only place I've seen sesame oil here was a tiny store down the block. (Tangent: mix sesame oil with soy sauce in a sauté pan and it's amazing.)
The reason we went to Jumbo was to buy some supplies for our upcoming trip: a tarp (to put over or under the tent in case of rain), bungee cords, rope. We went to three camping/fishing stores and none had any of these items. Neither did Jumbo.
We finally did find them at a nearby ferreteria - hardware store - that is truly a marvel of effective inventory management. The total walkable area of this store is maybe 2 square meters. There are no aisles. But behind the counter and all around the standing area are shelves packed floor-to-ceiling with every hardware item you can imagine. They had six sizes of bungee cords. I asked for rope and was given a wood pallet with rope samples in a dozen sizes. They had a painter's tarp which (with some duct tape around the edges) should do just fine to keep off rain. From a previous visit I know they have hundreds of sizes of screws and nails. So many choices!
When we were ready to check out, the proprietor summed up the amounts with a pencil and paper (the store does not have a cash register). And it was pretty cheap - 60 pesos for a whole bunch of stuff, compared to a simple extension cord I got at a megastore a while back for 50 pesos, that didn't even work.
Fruits and vegetables are a similar story — the supermarkets have a decent selection, but we can get everything we need (even limes!), plus fresh meat, at the verduleria on the corner two doors down from our building.