I bought a set of wheels for my 2002 Benz C32 AMG, and the prior owner had them customized with a paint that matches the centers of Benz AMG wheels offered on the SL65.  It's a dark pewter color.  The Benz dealer says that they do not carry touchup paint for these wheels, and they can't even give me any information on the color.

Is there any way to get this paint without taking the wheels to a body shop for manual matching?

Yes, the wheel paint colors are not coded anywhere so your choices are pretty limited. Without a color code, we have no way of knowing which color it is.