I'm not sure what you read in the wiki but a barber hone or any of the finishing hones listed here should work for you:

What hone(s) do I need? - Straight Razor Place Wiki

There are coticules listed in the classifieds:

Strops & Hones - Straight Razor Place Classifieds

A couple coticule related links:

Ardennes Coticule - natural sharpening stones from Vielsalm Belgium.
Welcome to Coticule.be

The experts will be by to give you more advice. I just wanted to throw a few things at you to ponder. If you'd paste the link to the ebay coticule you're looking at I'm sure someone can advise.

Good luck with your choice!