I bought Chomium Oxide powder from Kremer pigments, and am very happy with it. It is very fine, at 0.3 microns, as stated earlier. Here's order info from the US outlet...

Kremer Pigments
247 West 29th Street
New York, NY 10001
212-219-2394 voice
212-219-2395 fax
Kremer Pigments Inc. NYC
KremerPigments@GMail.com

Chromium Oxide powder
100 grams
$7.80 plus $9.50 shipping... $17.30

I also bought the Chromium Oxide "crayon" from Straight Razor Designs and am very happy with it also. 0.5 micron.

As for which is best, actually I am using both... StraightRazorDesigns' 0.5, followed by the Kremer 0.3. Inspection with a microscope, and the hanging hair test, both seem to indicate that there is a definite difference in grit size between the two, and that the Kremer does polish and sharpen a little finer than the 0.5 micron Chromium Oxide. But, truthfully, I can't tell that much difference when shaving. I got a good edge with only the 0.5 micron crayon, and I get a good edge with the 0.3 micron powder as the final polish.