Hi folks,
I have received very good value from:-
http://bright-components.co.uk/
Caps suitable as cathode decouplers, and some hi-voltage poly film caps useable for valve gear.
Antique Electronic Supply:-
https://www.tubesandmore.com/
Is based in the USA so we Brits have the penalty of postage from the land of Uncle Sam but they do have a large stock of hard to get parts geared towards guitar amp applications.