There are multiple chain sizes 1/8", 3/16", 1/4", 5/32", 7/32" and 3/8" chains so the /8 chain number has me guessing. After the qurestions below, here is my best guess.
Also is this for a chain grinder?" or
Are you looking of a hand held type stone?
Aluminum Oxide Chainsaw Sharpening Grinding Stone
or Ceramic stone?
I will assume that your looking for aluminum oxide stones at this time.
http://www.oregonchain.com/homeowner/products/accessories/replacement_stones.htm
This is wide selection of sharpening stones.
You can also use hand held chain sharpening files of the same size that have shapening angles marked on the file set fixture.
Hopefully you have a sharpening guide that will keep your angles correct while shapening.
A safety note:
Improperly sharpened chainsaw blades can be very dangerous to the user.
Thanks for using FixYa.
Kelly