I would recommand the smallest size Coticule you can still comfortably hone on. If you can hone on small stones you will save a lot of money. Also make sure to tell your seller what kind of Coticule you want e.g. coarse/fine, shape and maybe even which vein. Coarse Coticules are great in a progression but I would only use a fine one to shave of.

Buying one from ebay can be cheaper but you don't always get what you want.