This is something I'm planning on trying soon, so I don't have first hand experience. Any place can work, but I have heard in several interviews that locations near fruit orchards tend to produce the best results. Alternatively, you could simply harvest bugs directly from untreated fruit. My plan is to harvest directly from various berries I growthat ripen in different months, to see what is in my area during different seasons.
This won't necessarily be sour, and not every wild bug produces a significant amount of acid as a byproduct. Heck, not every one produces alcohol, either. But a lot of these wild yeast would probably work best in blends with other bugs that do produce sour beer.