I know that a lot of people hate Fiverr and find it worthless, but if you know how to review the websites properly, you can find some extremely cheap hidden gems on Fiverr. I recommend doing a group buy service to gain access to ahrefs or semrush to check domains and then contacting fiverr members to review their domain for their guest posting service. Also review the content on the blog itself. Most of them will be worthless, but the good ones you do find will be well worth the time, effort and money you spend on them.
Also, always create quality content. You can find reputable blogs via google that offer free guest posting services as long as your content is high quality and non promotional in nature. The secret here is to also have high quality blog posts on your website you want to link to, and build you guest post in a manner that links to your own blog post as an informational resource backing up a statement in your guest post. Link to a number of other posts on a variety of other domains that are not direct competitors as well and make your backlink to your site natural and non promotional and you are all set! The key is ensuring your content is high quality and your backlink is well disguised to provide valuable information in your guest post.