We recommend the following sites as a starting reference:
http://www.tubedepot.com
http://www.vacuumtubes.net
http://www.tubeman.com
http://www.audiotubes.com
http://www.thetubestore.com
http://www.vacuumtubesinc.com
http://www.tubeworld.com
http://www.worldtubeaudio.com
http://www.tubesandmore.com