For users who never "owned" Mavericks on their Apple ID, community-maintained archives are often the only remaining source:
: This platform hosts Apple OS X Mavericks (10.9.5) as a 6.1GB disk image for historical preservation. Mac OS X Mavericks 10.9.5 Free Download
The official way to download is through the Apple App Store, provided you have previously "purchased" (downloaded) it while it was a current release. While the full installer is no longer easily accessible for new users directly from Apple's main servers, various update packages and archives remain available. Official Download Options For users who never "owned" Mavericks on their
Before downloading, ensure your Mac is compatible with 10.9.5: Download OS X Mavericks 10.9.5 Update (Combo) Mac OS X Mavericks 10.9.5 Free Download