You don't necessarily have to find NOS tubes. Many old home stereos and plenty of
old military gear used tubes that will still test good if you take them down to an old
TV or audio repair shop.
So if you like to browse around at garage and estate sales, or GSA auctions you can find very high quality tubes for pennies on the dollar.
An example of a hot item might be an old Grundig or other european made
stereo that used Telefunken, Amperex, Mullard or USA brands RCA, or Phillips.
If you look at the particular makes and model designations at some of the NOS tube
supply stores, you'll get a good idea of what to hunt for.
Then narrow down your hunt for guitar amp related tubes.
12AX7A, 12AU7A, 6L6, 6V6, 6550, EL34, EL84, 5881, KT88, KT66 and so on.
As mentioned above some of these tubes are worth up to $250.00 EACH if they
test good.
The tube stores are also handy as a guide for learning the particular characteristics
of each tube as far as clean headroom or breakup factors.
Your hunt for audio tubes can also lead you to a virtual treasure trove of class A amplifiers.
Military spec gear also presents you with a possible killing on what you can get for your money.
Many of those old military spec components used all GOLD connectors.
A while back, I talked to an electronics liquidator.
They bid $800.00 on two crates of military spec components at a local GSA auction.
They ended up with $11,000.00 worth of gold from those components.
As mentioned in my Trash or Treasure thread, just this week, I found a rack of
perfectly good Mullard pre-amp tubes in an old POS Univox amp head.
The 12AX7A and 12AU7A tubes are probably worth about $40.00 each and I'll certainly put them to good use.