Hi,
From my 8 years experience....
Why you want to purchase such links? Pretty sure those sites will have a lot links and will be flagged by Google if not yet. It only matters of time when you get penelized for this. It's always better one good link then any paid links on any paid sites.... also backlinks don't inpact your ranking so much as it was like 3 ir 5 years ago...
Want good SEO? Then work on content you have. Make it as good as possible for your end user and everything else will come later.
Google is always looking for best content to they users. You can have bad seo but if content is good then it doesn't matter so much, you will be on the top....
Want to promote your site? Then do it via facebook, instagram and any other social media sites. You get followers, clicks, sales then it means your content is good enought to increase your ranking...
And most important. Don't forget that whatever you do, someone could do it better and that's why your ranking position is not going higher, then check what they do and do it better...
Hope that helps.