Wholesaling sellers list sources

[Liam McGoldrick]

I see a lot of websites selling leads for wholesaling. I am wondering what methods those websites use to generate the lists they sell. Are they scraping data from the web somewhere? I would like to try to generate some of these leads myself as an exercise. It may not be worth the time but I am really interested in how its done. 

Does anyone have any insight into how this is done?