tubebooks.org – Vintage info from the days of vacuum tubes: a collection of vintage engineering texts, vacuum tube datasheets, and other obsolete information, presented free of charge and without annoying advertisements.
AudioTubes.com (Brent Jesse Recording and Supply Inc.) – New old stock tubes, valves, tube testers and other geek stuff and very knowledable on tubes.
Patuxent Communications – Receiving, Transmitting and Special Purpose Tubes & Other Classic Electronic Parts.
The Tube Center – Buy, sell and swap vacuum tubes.
Old Guy Radiola – Sells tubes on Ebay. Very good prices and they are prepared to haggle and barter.
Tube Museum New York – Tested and graded tubes.
Angela Instruments – NOS Tubes and other guitar and amp parts.
TheTubeStore.com – Audio tubes for any amplifier from high-end home audio to classic guitar amps. Also a good resource for information on tubes.
TubeDepot.com – NOS and new manufacture tubes. They also stock electronic parts, books and amplifier kits. I built one of their Fender ‘Tweed’ amp kits.
Langrex – Electronic valves and components. Specialising in original British, German and American tubes.
Billinghurst Export Limited – UK supplier of electronic tubes and valves.
KarlTone – Valves, Speakers, Guitar and Pro Audio Accessories.