Since your iPhone 8 Plus has a sound recorder (also known as 'voice memo'), a very simple technique involves simply placing your phone near your computer's speakers, playing whatever song, music or sound you would like as a ringtone, and recording it -- you can even use your voice, your child's voice or even your cat's 'meow!' Record it with your mobile and there you go! Unless that function was specifically blocked on the iPhone 8 Plus, you should be able to navigate to the ringtones menu and choose your new recording as a ringtone. Of course, the quality may not be ideal but, after all, it's free!
Here are links to some of the free ringtones (iPhone 8 Plus-compatible) offered here at MartinsRingtones.com:
Free Horror MIDI Ringtones Free Tropical Ringtones Free Ringtones-a-gogo Free Techno Ringtones Listen To All Our Free Polyphonic Ringtones