The strongest link prospects are the sites already linking to several companies like yours. They have demonstrated both willingness and topical relevance, which is most of what makes outreach work.
Building the list
Take the referring domains for three competitors and intersect them. Sites appearing in all three are the highest-probability prospects available. Then subtract your own referring domains to leave only the ones you are missing.
Why this list converts
A site that has linked to three competitors is not being asked to do something unusual. The topic fits, the format fits, and the objection you normally have to overcome does not arise.
Look at how they linked
Before writing, check what earned the existing links — a resource page, a comparison, a quoted statistic. That tells you what to offer, and it is usually more specific than anything a generic outreach template would produce.
Try it: List Intersection (Common Lines) on SeoWolf's Notepad.