While it really depends on just HOW fine and polished you want to make your edge, any old sharpening stone would do for ya.

I've gone through a few of these over the years, it's not so abrasive that you'll grind away half your knife by accident, but enough to make it work. https://www.amazon.com/King-1000-Grit-Whetstone-Plastic/dp/B000OT1ZOC/ref=sr_1_15?ie=UTF8&qid=1481149234&sr=8-15&keywords=waterstone

I cannot recommend the double sided stones with the low grit on one side, high on the other. The grits get washed together and it kinda ruins both stones pretty quick.