The source is closed for uTorrent and has always been. The fact that BitTorrent is now a re-branded uTorrent makes it also closed, even though they closed it a few years back.
Therefore if you are looking for CURRENT source to BitTorrent/uTorrent you would be doing so illegally.
If you are looking for archived source for versions which were previously released as OSS then your best bet may ironically be the varying search mechanisms for torrents.
