I found this single speed kit for Campy but it does not include a cog: http://www.universalcycles.com/shopp...&category=2621

From my searches it looks like you'll be stuck buying an entire cassette then taking it apart to get a cog: http://www.universalcycles.com/shopp...48&category=42

Ebay may be a much cheaper option, or try and call some bike shops and see if they have any used 8 speed cassettes laying around. Most only have a few worn cogs anyway.