Hello 1&All i  have not had any tube equipment since the 90s with that said i just upgraded my PreAmp  with a  NAT - Symmelical  All Tube PreAmp just purchased on Audiogon  not knowing  the age of this unit or who manny hours  i have been playin since 10;00am when it arrived till 6;00pm i am very happy with this preAmp Replacing  my Plinius 16 which now goes  4 sale soon asking 1.400 .00  any way i need advise on buying / replacing  all the tubes such as 6n30p-dr ,6n2p-ev,6n1p-vi , 6x4wa, oA2Wa Looking for Really good QTLY Tubes any Help PLEASE & thank you G
Tube Depot, Viva Tubes, The Tube Store. Between these 3 providers, the tubes needed should be available. If it’s some odd tube, like ones yours truly recently needed, an order was placed to a man from the Ukraine on that auction site. 
Also, channels need to matched… so pairs, quads, need to be matched. There is a small charge to make sure the output is matched.
why buy new tubes? many, most last 10,000 hrs.

I just had my McIntosh mx110z tube tuner/preamp overhauled by Audio Classics. All 17 vintage tubes tested good, he left them all in.

You don’t know about the tubes that came in it right?

I advise tube users to buy a simple tube tester, which gives you knowledge, confidence, and, when trouble occurs, quickly find or rule out the tubes as the problems, as well as test what you buy, see how matched they are. My little tester always agrees with my big heavy one, I don’t bother with the big tester

You need to verify it has sockets for your tube types, and have the manual/chart/tube book to know the settings

like mine:

don't let the rusty hinges scare you, they used poor quality metal to keep costs down.

Elliot is correct. These tubes are small, they last multiple thousands of hours. Particularly when you first get into tubes it is a little disconcerting. It is power amps that have shorter a tube life. But with modern equipment even these are getting longer. It is likely you will not face any tube problems soon. Personally, I would either not worry about it and just order a full complement if something fails or order a full complement and put in the closet. I have quite a pile of tubes I ordered just in case and traded the equipment before a tube went out. 
…..well the Mac gear from the 60’s is known to be very gentle on tubes, don’t assume every designer this astute……and yes, i have both a 1961 MC240 and 1965 MX-110Z…..both w mostly origional small signal tubes….but not all, your milage may vary….

Mac was also thru the seventies famous for walk in performance clinics…mostly power output tubes got changed….but some small signal as well….

enjoy the journey 
be cautious

you can ’maintain’, ’improve’, or ’RUIN’ the sound of excellent tube equipment, or go broke chasing nirvana blindly.

you want to have a spare or two on hand, matched pairs are nice, however, tubes do sound different, so if you like what you are hearing, perhaps spares of that brand/model.

tube break-in: In, no signal, leave on for 60 hours, done, now listen.

tube source: Brent Jesse let me swap, after 60 hours break-in, and gave me a refund for the price difference, all I paid to switch was the extra shipping.

tube bias: some equipment has easy and safe adjusters, even meters, mine has internal adjusters, hard to do, danger, potential harm to me and/or the unit. Bias is not critical IMO, if it sounds great, bias is smart, but I never bothered, may or may not bias my Cayin.

for instance: My Cayin Amp.

1. Came with 6550’s (seller said biased for them). Sounded great, after a while I chose and swapped in some KT88’s. I prefer their sound (bias unchanged).

2. came with 4 absolutely identical looking 6S tubes, but actually 2 6SL7; 2 6SN7. They were marked properly, but somehow I put two in wrong place, sound got awful, balance way off. My fault, blew a 10,000 hr tube in it’s near infancy I bet. A used tube from my mess-o-tubes (cases, boxes, drawers), sounded great again, but not matched.

Picked a pair of expensive highly rated ones (thinner/taller/chrome tops, never repeat that mistake). Hated the sound, broke in, still hated the sound. I would have rejected the Cayin if it sounded like that. Picked new ones like my old used one, they sound great. It could have gone from bad to ok, or bad to worse, I felt and still feel lucky.

be cautious.