For my type of business, which is more B2B, cold calling and cold emails have worked the best. Both are free but take some time to set up and tweak scripts/emails, not to mention the time it takes to cold call as well. In the future I'm going to be running a few thousand in Facebook and Google ad spend.
Lots of lead scraper websites online, D7 leads, KleanLeads, Apollo is a big one. My niche is local businesses however so I just go to Google maps and compile a list manually although there are some Chrome extensions that just scrape it for you for free. Then I cold call myself, yeah.