Downloading MainStage 3.6.6 for compatibility with older school computers

Looking for a way to download 3.6.6 - I work with a marching band and the school computers are old enough that 3.5.3 is the last version for them and they will not open 3.7 files but I can open/save on a secondary laptop in 3.6.6 and work my way backwards

[Re-Titled by Moderator]

Original Title: mainstage 3.6.6