Sinks have a pretty decent chance to drop acid. This makes houses and resturaunts prime places to look due to kitchens and bathrooms always having sinks. Also, larger POIs with multiple bathrooms with multiple sinks are good areas to look. I typically find 20 jugs of acid for every wheel I find. However, if you're completely against crafting them for whatever reason, Pass-N-Gas usually have at least one car parts box, sometimes garages in homes will have them. They might pop up in a few other random places.