I agree with the above comments saying it depends on the specific tube and the application. There are some very rare and high priced tubes, but overall there is no problem finding great sounding tubes. It's been my experience that price doesn't fully equate to performance with NOS. A Bugle Boy or Mullard may be at the apex, but a Brimar, Mazda or even RCA at a quarter of the price can be excellent tubes.
IMO, unless you are buying on Ebay or scavenging electronics at estate sales, you don't need a tube tester. Getting an accurately functioning testers and knowing how to properly use it is a non-trivial task. A good vendor can do all that work for you. I'll plug Brent Jesse, but he's not alone.