I have personally used the MX-1proDV with very, very nice results. It is less than $1,500 brand new and will do the exact job that you need perfectly. You can ad the new "multiplier" to add more cameras, but I haven't received pricing info yet.
As to finding them used, no idea. I'd try
http://www.ubid.com, ebay, and videouniversity.com in the forums section. Outside of those ideas, I'm no good at finding used equipment.
Michael