SourceForge or GitHub), or given to selected game community members, or sold with the game, or become available by other means.
The source code may be pushed by the developers to public repositories (e.g. Such source code is often released under varying (free and non-free, commercial and non-commercial) software licenses to the games' communities or the public artwork and data are often released under a different license than the source code, as the copyright situation is different or more complicated. In several of the cases listed here, the game's developers released the source code expressly to prevent their work from becoming abandonware. When there is no more expected revenue, these games enter the end-of-life as a product with no support or availability for the game's users and community, becoming abandoned.
