Content Explorer has a searchable database of 404 pages that have backlinks. If you add the right filters, you’ll find tons of opportunities pretty fast
Just do this:
1. Enter a keyword into Content Explorer
2. Filter for “Only broken”
3. Set a referring domains filter for a minimum of 50
This will show you some 404 pages that have at least 50 Referring Domains pointing to them. Then you'll have to find the emails of the websites and use a template from many that are out there for broken link building.
I always have a VA of mine doing this regularly for sites of mine and I get some pretty good links for a cheap price.