You should try craigslist dot com
Personally, I just bought a brand new 200g oceanic tank with stand and canopy from another member of my local marine aquarium society for $650. Unbelievable deal. I strongly suggest you google all the major cities near you and the words "aquarium", "reef", and "society", and at least make a "guest" account, even if you don't want to pay the dues to join as a member.
Hope that helps! Good luck in your search!