Post a message in this thread on Bimmerforums to solicit potential sellers:
The UNOFFICIAL E39 Touring Thread! - Bimmerforums - The Ultimate BMW Forum
Also, cruise the For Sale section here:
5 series & 6 series - Bimmerforums - The Ultimate BMW Forum
I shopped for a little over 3 months. They are around, but the good ones go quickly, particularly the manuals. I never thought I would say this, but I love my wagon. For the price, it offers great value, luxury, and practicality. Doesn't look half bad either. Good luck with your search.