Apple just released the first public Beta of iOS 12. Committed Apple users can download the Beta for free from Apple's site, but be warned - as beta software, there are quite a few bugs still being worked out. Still, this is the only way to get some of the new features in iOS 12 for free.
https://www.notebookcheck.net/First-public-beta-of-iOS-12-released.310998.0.html