This is a legitimate question--but what if you need to support different versions of open source packages and you need to be sure that they will exist years lat

This is a legitimate question--but what if you need to support different versions of open source packages and you need to be sure that they will exist years later. This is something that I'm facing now in that I may be using numpy 1.2.x, but suppose someone else is using numpy 1.3.x and we all need to play well together in a distributed environment--the context needs to be stable. If I don't put this in source control, then where's a good place to stash it so that I can reconstruct the various contexts when I have to migrate to a different server?