Parts Center and Boiler Spares Direct are some of the websites where you can find spare parts for gas boilers. These places feature various types of gas boiler spares from different manufacturers.
Parts Center features domestic gas boiler spare parts complete with descriptions, and the site has a list of its entire gas boiler spare parts manufacturer and provides you with branch locator option to find the store nearest to you.
Boiler Spares Direct gives you various spare parts categories to start your search. This site provides photos of all gas boiler spare parts in stock with their specifications and prices. The site also provides information on the suppliers of the available gas boiler spare parts.