"I'd email freemans cycles - http://www.freemanscycles.co.uk, as they stock one size, and might know where to find another size.
Other than that I'd expect to be doing the check ebay regularly / go to swap meets and eventually get lucky.
Do you sit on tensioned leather? Is yours a lugged steel bicycle? Is carbon just that stuff people say we need to stop putting into the air? This is a group for people into vintage bicycles. (typically pre-1980's)See More