Whitehat SEO That Feels Blackhat
Your competition is probably pretty lazy on the web. Many website owners buy website links and engage in shady SEO practices. You risk a lot by going blackhat. Thankfully, none of that is necessary. There's a very whitehat method that feels so wrong, but is very ethical and very powerful.
Old sites that are no longer kept up in good working order usually contain a lot of broken links. Geocities is famous for this kind of thing. All of these dilapidated websites used to have good content on them. Special reports, well-researched articles, and sometimes even video. When the website goes offline or is not well maintained, and website owners change permalink structures or just abandon the site, it generates a lot of 404 pages.
These 404 pages are forgotten pages that never got redirected using a permanent redirect. These 404 pages just clutter up the web until they are eventually deindexed. Find these broken websites and drop the 404 page URL into the way back machine. Find out what used to be on that page. Then, figure out who is still linking to that stale old website.
Contact the people linking to that page with an offer. Freshen up the content that used to be on that 404 page. Don't steal it. Make it better. These website owners won't want to keep linking out to a broken website when there's a perfectly good website - yours - that offers fresh content and offers something valuable to users.
The likelihood that they will link to you is high. After all, they were already linking to the content on the 404 page. If your information is an updated version of that 404 page, why wouldn't they link to you? These website owners will probably thank you. Imagine that. They will thank you for the link to your website. It feels blackhat, but it's not.