inna,
larryi makes a good point, and that is why I usually only but from Andy at Vintage Tube Services. He tests every NOS tube extensively on a variety of parameters on testing equipment that he owns that most tube sellers don't own. There are exceptions. I did buy a couple of fantastic Telefunken 6922 tubes from Upscale Audio that were matched (hopefully).
To answer your other question inna, Andy, and other dealers I'm sure, travel to auctions and estate sales etc. to buy lots of tubes that can later be evaluated for sale. Andy was just going to one when I spoke with him a few weeks ago.