I don't know if Magnaflow makes an E36 cat-back. If not, you would in fact need to have a muffler shop custom fab the piping. That said, I don't like how E36s with Magnaflows sound on YouTube, and would recommend getting one of the other cat-backs mentioned above which require no custom work and you can install yourself using basic hand tools.
1998 BMW 328is
1966 Pontiac GTO
2007 Subaru Impreza 2.5i 5-door
View my photos: Caught in the Wild