Q: Is there any way to get the source of System 7

Apple's System 7 OS has not been released as open source software.

Apple has removed their links for older software such as System 7 even though it had been available for download as a complete OS.

However, it (and other stuff no longer listed by Apple) can still be downloaded from places such as this-

http://ftp.iinet.net.au/pub/apple/US/Macintosh/System/Older_System/

Note that this site is neither Apple approved nor vetted. That said, it has been used as a source by many folk with no negative feedback.