Spoiler Alert, click show to read: 

Neb's house is the one in the top left corner of the bridge district, and locked.
Note you'll only meet Neb and get the Illithium off him if the temple quest for the statue is active (or at least that's how it works for me usually).
Also, doing so will cause you to 'fail' the temple quest as the sculptor will see that you brought him fake Illithium. Still, the person at the temple will be happy with the fake illithium and you get slightly less experience, I think.