http://www.seaerospace.com
is a great place for used avionics. All the right paperwork too, if you ask for it.
Just remember that avionics shops hate this stuff. The trade margin they make on the supply of the new item is as important to them as the installation cost. So check with your hopefully friendly avionics fitter before you do this.
The GNS430 doesn't do more than a KLN94. It's popular because it contains an FM immune radio/VOR/ILS, and its moving map display is slightly less useless than that of the KLN94
