You might want to check out our friends out in the Far East...

Japanese sites have a lot of mech/robotic treasures!

To start you off on your quest, here are a couple of sites, follow their links to other sites and you will find all kinds of great ideas...

A personal favorite, he creates plastic models, but they are awesome!!!
http://aswars.tripod.com/index.html

More of what I glean for my own Space Warship artwork:
http://yamatomechanics.org/

Run around the Japanese sites with your Broswer set to translate (IE right-click in the browser window and "translate page to english" option)

 

Carrara 7 Pro, Anime Studio Pro 8, Hexagon 2.5, Zbrush 4.6, trueSpace 7.6, and Corel Draw X3. Manga Studio 4EX, Open Canvas 5, WACOM Cintiq 12WX User