There are a few places online that sell them, let me get you a link.
If you're in the southeast (alabama in particular), there is a nice organ store in Gadsden that sells vintage Hammonds...check your local papers classified ads, I've seen a few A100s and C3s being sold by churches...and there's always ebay...
http://members.aol.com/foto222/picpage.html#list - They have one B3, with a 122 leslie...~$10,000
http://www.keyboardexchange.com/inventory.html - This is a pretty neat site, they do some custom work. There are two B3s (with 122s), for 7k and 9k.
http://vintageorgans.com/inventory.asp - There are quite a few vintage Hammonds here (B3s, A100s, C3s, etc).