Broken link building is to find broken link in your competitor sites known as 404 pages. Identify them and get the webmaster to point them to a relevant page on your site.
First install chrome extension – Check My Links.
Find resource pages using few search strings using google search.
Search string you can use –
keyword + intitle:resources
keyword + intitle:blogroll
site:.gov keyword + inurl:links
Then put the sites that you find there on a sheet. And check them one by one with the extension for broken links. And when you find the broken link you can send email to the webmaster requesting them to link your relevant content to their broken link. You can create better content by review their old content.
To get the old content on the broken links you can use www.archive.org