In no particular order (and implying no particular endorsement) here are some places from which to buy tubes. Most (or all) of these places also stock a good supply of capacitors, sockets and other components typically found in tube equipment which may be hard to find at a "modern" electronics supply house like MouserElectronics or DigiKey.
TubesValves ~ Cape Canaveral Fl. USA
World Tube Company ~ Tacoma, Washington. USA
Triode Electronics (?NedCarlson). Chicago, IL. (Don't buy on this one, I have been waiting 2 months since I bought 3 transformers and they have sent nothing yet and they don't answer my emails anymore)
AntiqueElectronicSupply. Tempe, AZ.
New Sensor. New York, NY.
RF Parts Company. San Marcos, CA.
Michael Marx. St. Charles, MO.
Thetubestore.com. Hamilton, Ontario, Canada
Vacuum Tube Supply. Mark Frasier. Johnstown, NY.
The Tube Center. Orlando, FL.
ESRC Vacuum Tubes. Orlando, FL. (Seems to be the same place as "The Tube Center")
You might also consider buying tubes from you favorite HammondTechnician.
Vintage Hammond. Houston, TX.
Tejas Tubes Houston, TX. http://www.tejastubes.com
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