Look out for Tregar dairy products. They tend to be on sale in the smaller shops rather than big supermarkets but if all else fails they are listed on Mercadolibre. Much better quality, IMO, than Serenisima etc and generally a little bit cheaper too.
Casa Munay on Scalebrini Ortiz near Guatemala. I think their deliveries are on Sat and they have product through mid week. Sometimes they have frozen butter that they will sell you if you ask for it. It's perfectly good and defrosts quickly. I just bought a frozen log of butter that weighs about a kilo. I cut it up with a hot knife as needed. They usually have yogurt and milk with full fat and no fat. Full fat taste way better.