I collected all the tubes I'll ever need two/three years ago on eBay, online dealers, local electronic surplus stores, etc. I even bought an used military TV-7DU tube tester and had it recalibrated.
As the supply is depleting, you will have to pay a lot of money for good NOS tubes. Kevin Deal's a good source, but I'm not sure if he's got that much in stock. Check out www.audioasylum.com's homepage and the FAQ section. There are some reputable dealers there. If you are currently using Sovtek, you will hear the difference even when you just get the Sylvania JAN tubes, or the Russian NOS tubes (I think it has the designation of 6h-23N). These are much cheaper than Amperex/Siemens/Mullards, and much more readily available.