Since your iPhone 7 sports a sound recorder (also known as 'voice memo'), a very popular technique involves just putting your phone near your computer's speakers, playing whatever song, music or sound you'd 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 phone and 'voilà!' Unless that function was specifically blocked on the iPhone 7, you should be able to navigate to the ringtones menu and assign that recording as a ringtone. Of course, the sound quality may not be perfect but, at least, it's free!
Learn more about making your own free ringtones
Here are links to some of the all free ringtones (compatible with your iPhone 7) we offer:
Free 'Unclassified' Ringtones Free Tropical Ringtones Free Ringtones-a-gogo Free Techno Ringtones Hear All Our Free Polyphonic Ringtones