Litchfield Station http://www.litchfieldstation.com/xcart/home.php
Tony's Train Exchange http://www.tonystrains.com/
DCC Train http://www.dcctrain.com/shop/category.aspx?catid=3
M B Klein http://www.modeltrainstuff.com/
Southern Digital http://www.sodigi.com/
There are lots more.
TCS at http://www.tcsdcc.com/
They have quite a few installation photos for many different locomotives.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.