This guy could be just what we're looking for...

Here's a lapidary store that sells high purity lapping products, in reasonable sizes (you don't have to buy two pounds..) and for reasonable prices.

he has:

Chromium oxide (powder) 4 oz. for $4.50!!!!! great price... I'll email about the purity

cerium oxide 2oz for 8.50, and this is white.... not yellow. Apparently the white is a higher grade than the yellow? I'll email... :)

he also has Linde A, Linde B, etc.

Check it out here:
http://www.myrockhound.com/toolssupplies/polishes.html

just be careful some chro2 is not clean as they should be. Prices is really great.

I can't say about the purity of their Chromium Oxide, but that is the same guy I buy my Linde A compound from.....the Linde A is exactly the same as the stuff I have gotten from other places in the past.