You can get it on Steam (I recommend this, great speeds) or pretty much everywhere else.
I'll list a few for your convenience.
http://www.fileplanet.com/promotions/bioshock/
http://www.gamershell.com/download_20697.shtml
And also a Piratebay torrent
http://thepiratebay.org/tor/3778797/BioShock_PC_Demo
Almost 1000 seeds, should give decent speeds if you've got your ports forwarded.