One of the major manufacturers is Wilks
www.wilks.co.uk -- you could try them although I don't knw if they sell direct to the public.
Henshaws
www.henshaw.co.uk stock and sell a variety of parts and materials for tube repairs and are always very helpful.
Other possibilities would be the manufacturer of your RIB, another local RIB manufacturer, or one of the ther tube makers.
John