I thought it was Cr2O3?
anyway, yes you can get chromium oxide for cheap. I got a brick sized piece which is used to load polishing steel on a buffer wheel.
But the grit size is 15 micron instead of 0.5 micron, so it is completely worthless for the purpose of polishing a razor's edge. I suspect the same is true for artists pigment.
The tiny tubes contain only the chromium oxide in grease. It's just a LOT finer than what is used in other applications, and thus more expensive.