That depends if you are into the 460 and 461 range of the G or if you are aiming at 463.
Pointedthree.com has massive knowledgebase of everything to do with the 463. You can find some 460 / 461 things, but their strength is in the comfortable vehicles.
Clubgwagen.com has a great selection of 460 / 461 vehicles and everything you need to know to build one from scratch. You also have 463. This forum have some modified vehicles, both in suspension and driveline.
Recently there is a new and upcomming forum. Started in UK & Ireland, they have people signing in from all over the world. Some of the members do very intersting builds. Not bling, but down-to-earth vehicles that you use in real life. Gwagenregister.com