Try here:
http://www.uniform4sale.co.uk/raf_main.htm
To quote "This site is dedicated to the sale of military uniform on behalf of serving Royal Navy, Army and Royal Air Force Officers, Warrant Officers, Senior Rates and SNCOs who have an item(s) of uniform that they no longer wear and would prefer to sell rather than throw away or allow to take up closet space collecting dust."
There used to be another site at
http://www.kit2x.co.uk but that seems to have gone, unfortunately.