VMware doesn't appear to be selling Fusion licenses for version 10 any more. If you have a VMware Customer Connect login, you might be able to open a non-technical support case with VMware and ask if they'll downgrade a paid version 12 version to 10.

I know a Fusion 12 paid license can be downgraded to Fusion 11 through the Customer Connect portal.  You can't downgrade the free personal use version of Fusion 12 Player to Fusion 11 because the free player licenses did not exist for Fusion 11.

You can use that downgraded license to install Fusion 11.5.3 (which was the last version to support High Sierra). Your existing virtual machines will continue to run. (although I'd recommend you upgrade the virtual hardware version and install the version of VMware Tools that is provided with Fusion 11.5.3).