Matthias, the black chassis were primarily used in the ready to run cars and I believe they predate the silver/grey version. There is no real difference between them, in any case. The black one is often more expensive, because its not as common.
Best of luck with you project!
MoralThreat, try RCMart.com or one of the other HK shops (rcmodel, rcmushroom, stellamodels, vellrip...etc).
Jim