There are two compounds that maybe being referenced here. Is he indeed looking for heatsink compound, or what seems a more logical thought it is thermal silicone rubber... the type dispensed as a jell-liquid, like silicone caulk, it's widely used as an assembly glue in high heat situations... a particular formulation is even sold as engine gasket sealent material.
Most heatsink compound does indeed have supended metals... but then so does some of the thermal silicone rubber... supended copper generally. Is the later indeed what your looking for? If so try the auto parts store. But check the temp range on the package those formulations may require quite a bit of heat to "bake" and form the intended thermal bond. You may have to order it to get the right formulation... there are many.
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crashK6