Older versions of Photoshop

This OS version should support CC2018 as per the following link

https://helpx.adobe.com/photoshop/system-requirements/earlier-versions.html

You can try and download from the following website. If the links mentioned on the website don't work, then it means that Adobe has discontinued providing the download and genuine installers are not available anymore. Be circumspect if someone contacts you claiming to provide an installer in such a case.
https://prodesigntools.com/adobe-cc-2018-direct-download-links.html
-Manan