The first thing I would recommend is looking up what websites are currently linking to your site. I have experience using SEMRush and they have a tool called **Backlink Analytics** that will tell you when a site linked to you, how many links, and how long ago did they link to you. From there you can reach out to them to link specific content from your site.
Also, you’re able to look up your competitions backlinks and reach out to those websites that are currently linking to them. Or, if one of your competitors is no longer in business you can run a backlink search on them and find out which backlinks are coming back as 404 errors.