My overarching question is: What's the best way to source leads?

To get into specific tools, I've seen numerous services to find businesses and contacts mentioned in the message boards. I'd like to know if any of them are actually useful, even services not mentioned here.

Zoominfo
Clay.com
Sourcescrub
Grata
Udu
Inven

For a previous search, I found 250+ leads through public records because the companies all do business with the City of Denver.

Even with the list in hand, contacting and following up with everyone was very time-consuming. The search resulted in few good leads and no acquisition. I'm expanding my search criteria but would like to find a different approach – especially for companies that don't have contracts with public entities.

Is there a better way of going about this?