How do you find a lead for doing outbound

Finding a lead is one of a tough process. It's always painful for me to find a lead for my domains. So far I found this method very helpful for me.

Google:- A simple site:whateveryourdomain can give you some lead

Social Media:- I find leads using Linkedin but not free after one month as you need pay in order to contact end-user using their In-Mail feature.