Customer Discovery

Hey guys! I wanted to get some feedback from you guys about customer discovery. We hear about "getting out of the building" pretty much every day when it comes to customer discovery. But how exactly do startups find the right people outside the building to collect data? I've ran into problems like this, and I always wonder what other people do if their customers are hard to identify on the streets, like actors, landlords, or stay home moms? I've also come across some data marketplaces where you can purchase data (which I assume is probably outdated). And without spending a lot of money doing surveys, has anyone discovered an efficient method?