Freelancers hang out on sites like Upwork.com, Freelancer.com, and other similar sites. You could build a simple scraper and scrape the list from these sites.

This is a great resource of ideas to help freelancers find customers: https://tiiny.host/75-ideas-freelancers-agencies/

I am not sure.

I think Slack and Facebook have groups for them for sure,

and I got some results by searching "Freelance developer" on Linkedin.

I'm working on a software which helps freelancers/small business owners increase lead generations through their own website. How can I get their list?

How to find freelancers/small business owners?