How are tubes to be purchased in general?

If you are careful and don't do anything stupid (hooking up speakers to a powered up amp...) then they will last you years. I have a bunch of amps with very old tubes. My EAR amps got 10 years out of the tubes without ever blowing them. After some time, the sound lost some dynamics and it was easier to distort at loud volumes. I tested the tubes, ended up replacing them for very little and expect many more years of use. Try www.tubesandmore.com or www.thetubestore.com to get an idea on tube pricing.
By the way, it is best to replace worn output tubes in pairs/quads. You also want to make sure that both channels have the same quality tubes or you will notice a difference. Tube biasing helps but, if you push them hard the older tubes will stand out.

i'll try and help--i dont own tube hifi but ive owned lots of tube guitar amps over 20yrs..
if a output tube goes early on deffo check the bias and change the tubes in a QUAD i wouldnt just change one---if ya cant find a quad buy 2 matched pairs and put one tube from one pack in first then next to that put one in from the other pack soforth---i change my power tubes once a year frequent use if i use the amp more than usuall in a year i change them in 9 months time.......
pre amp tubes last a lot lot lot longer so you probally wont want to change them unless you feel you must(anyways they aint exspensive unlike pwr tubes)
check out a guitar magazine/paper called ''vintage
guitar'' and there are so many ads from places that sell all sorts of tubes in quads pairs ect JJ,SOVTECK,SVETLANA, ECT ---in my own experience i thought the russian tubes sounded a bit better than the chinese tubes like RUBY...
some help maybe..cheers

It is usually best to replace tubes, in the units, they are being used. ie: pairs, or quads. It is a push-pull circuit, so replacing the pair, would be best. Tube matching, in power tubes, is important, because of the nature of push-pull designs. If you replace one, which doesn't match, it will unbalance the circuit.

As mentioned above, if your amp is self biasing or has individual bias pots, you don't have to get matched tubes, though it does not hurt as it may help to assure you that there is nothing whacky in the way the tube measures.

If the amp is cathode biased like the Rogue I prefer to use matched sets. If you have an amp with one bias adjustment for each pair or quad you really should use matched sets.

The issue here is how the tubes idle...it's like having four carbs on a car...they should be synchronized. If one tube is a current hog, it will run too hot in relation to the other tubes and fail early.

If you need matched sets, when buying a re-tube...ask the tube seller to give you the measurements to refer to if you lose a tube. That may help. Though most tube sellers matching rigs do not have regulated power supplies, so the batch they test six months later when you call may have a bit of a variance from what they tested for you.

That's why we had to get our test equipment custom-built. Our single tube tester is regulated.

Our new tester was built by Eric Barbour, the former Chief U.S. Engineer fro Svetlana Electron Devices.

The Hewlett Packard Power Supply we use cost more than entire test apparatus that are sold. Together between the two racks it will test/burn 40 tubes and weighs over 200 lbs.

I don't know why the commercially available test rigs don't have regulation. Even the Sophia curve tracer does not last time I read. It would raise the price, but makes for a really usefull tool.