How are tubes to be purchased in general?

Agree with the posts above. Nice to measure the voltage (bias) running in your tubes - get a meter. The only time you are going to run into trouble is if your tubes are running hot and some amps change over time. Not like Sand amps where you plug them in and never think again for 10 years. Get to a local club or post on the audioasylum.com tube section. Someone will have your amp and will be able to tell you just how to take readings and what they should be. I think you will find the audioasylum site invaluable if you are new to tubes.
Good Listening!

Over the last 7 years I've owned two different all tube setups and I've had it pretty easy when it comes to tubes. I've never had a failure and the tubes were not expensive. I have Rogue equipment now and the amps are auto biased (you don't do anything). My only qualm with tube amps is that they produce a lot of heat in the summer. A small price to pay. Btw, I just purchased two quads of Electro Harmonics 6550 tubes from Triodeelectronics.com for $99.90/quad as an upgrade. The original tubes still work fine. I'll test the new tubes in a year and if they're ok then keep on truckin with them.

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.