Install Homebrew (old version) or MacPorts for legacy open-source software.
When downloading Mac OS X 10.6 Snow Leopard as a DMG file, consider the following: Mac Os X 10-6 Snow Leopard Download Dmg
(e.g., ClamXAV) before opening.
Sites like the Macintosh Repository host various versions, including the 10.6.3 installer and specific builds for different hardware. Important Installation Notes Install Homebrew (old version) or MacPorts for legacy