It's at <ftp://24.0.172.162/pub/RM/spcaa.rm>.
There are a lot of other South Park movies, both in RealVideo and Quicktime format, at <ftp://24.0.172.162/pub/>.
If someone would be so kind as to provide a RealVideo copy of the original unaired pilot, I'd be glad to put it up on my FTP site and let everyone download it!