[Robert11]

I am attempting to locate a source for Diamagnetic Pyrolytic Carbon Powder. 

Preliminary needs for testing require less than a kilogram.  Production needs will depend upon efficacy and pricing.

[Enthalpy]

Good that you need less than a kilogram, because the growth methods tend to need days for a thin layer.

Pyrolytic graphite uses to be diamagnetic, no worry with that. The shape is naturally the one of the mandrel, with carbon making a thin cover later separated from it, so a powder would only need to crush it...

Pyrolytic graphite is very anisotropic. You're sure a powder, which isn't oriented generally, will bring you the desired properties, aren't you?

Of course, you have deep pockets, because a lengthy production process at high temperature can't be cheap.

Graphtek LLC and Momentive supply such things, among many competitors. Pyrolytic graphite rather than carbon; beware they may well differ, I didn't re-check.