Find out the make and model of your boiler and enter this in a search engine in quotation marks followed by a plus symbol and the word fan.
You will get dozens of hits most of them mail order - I just did it for my boiler entering: "potterton profile" + fan (972 UK responses on Google).
Alternatively use yellow pages and call a plumbing supplies business in your area.
Peccavi, December 2008
