I've had 3 fully remote positions in my career so far and I found them in the following ways:
1) filtering listings on AngelList ---> the job before last
2) searching "remote" on the "Who is Hiring?" threads here of HN ---> my last job
3) looking at the job listing page of companies whose work interests me who I've heard have remote-friendly or -only cultures ---> my current job and most of the places I'm currently applying
https://remoteok.com
https://4dayweek.io
https://github.com/lukasz-madon/awesome-remote-job
https://weworkremotely.com
https://awesomejobs.io
https://remotewide.co/
(not op, no affiliation with anyone on this list)