How do you test different tubes?

While there aren't any tube lending libraries there are tube dealers whom people have learned to trust like Jim McShane, Kevin Deal, Brent Jesse and others. They can't tell you which tubes you would like in your equipment but they test the tubes, stand behind them and can give you general guidance about what might work for you. Here's one thread that's a few years old but should give you someplace to start.

Andy at Vintage Tube Services has the real deal. The idea that there are no good NOS tubes out there is patently untrue. As Elizabeth indicated, many private sellers are offing ones they did not like, but the best dealers are getting ones through means others do not have.

The last place I would buy a tube is eBay FWIW. At least not at full price as you will need to sort them.

I just spoke with Brent Jesse (audiotubes.com) regarding NOS 6SN7 tubes. He has an impressive supply of very high quality NOS tubes of all types. He allows you to buy, try, and return for upgrade or refund (-20% restocking fee). Although I didn't end up buying from him this time (I found what I wanted locally), I plan on doing so in the future.

I concur that there are legitimate vendors as named above along with the Tubedepot & Tubemonger that you can get quality new in box NOS tubes, and at reasonable prices which normally to my ears sound a lot better than the new production.