The easiest backlink I ever got: I found a dead link on a popular blog. Emailed the editor. Pointed to my article as a replacement. Got a link within 48 hours.
How to Find Broken Links
Use Check My Links Chrome extension. It is free. Browse any resource page or blog post. It highlights dead links in red. Each red link is an opportunity.
The Outreach Email That Works
Keep it short. "Hi, noticed your link to [X] is broken. I wrote something similar at [URL]. Feel free to use it as a replacement. No worries either way." No pitch. Just being helpful. Works 20% of the time.