A guest post is gotten two ways... hard work or hard cash... and sometimes a combination of both.
If you want to get them for free than I suggest starting a direct contact campaign... throw up an email server with iRedMail on a vps and throw in mautic... For the frugal you can add a Gform to Gsheet setup, or just an ssg site with a phone number. If you are willing to hit an extra few keys, install wordpress for a shingle site and forms for your link transactions. If you don't mind the CLI, installing drupal will even do you better. Don't be afraid to make/receive phone calls or write a letter, though paying for postage can suck.
Or find PBLNs and pay cash... yuk.