I use the diamond stones - this is the best price I found:
http://www.knivesplus.com/DMT-D6C-DMT.HTML
I have the coarse and the fine. The coarse is good for serious nicks or bevel changes. The fine is great for touch ups. After the fine stone I use leather to do a final polish. If you get diamond stones - get the Monocrystalline diamonds not the poly. The mono last forever, the poly break off.