Believe it or not, we get asked quite a bit where one might download the old WPMU core files. There are many sites that have not yet upgraded from WPMU 2.9.2 or are running on even older versions of WPMU. The mu.wordpress.org page directs you to what seems to be the regular WordPress download page. Click on release archives. Now, scroll all the way down to the bottom and you will find old MU releases in the last section:
You can download a zipped file of any of the old mu releases that you need.