It seems Apple started selling System software with 7.1, so I'm going to be so bold:

ftp://ftp.cs.tu-berlin.de/pub/mac/sys/os/us-english/[for System 6]

http://macintoshgarden.org/sites/macintoshgarden.org/files/apps/System_7_0.smi__0.sit[for 7.0]

These presumably still fall under the original license agreement.