You're pretty much faced with a limited array of resources when it comes to getting your hands on one of these things, and online auction is easily the most accessible market, for all its flaws. Some other avenues to explore are surplus dealers, classifieds in radio/electrical engineering communities, estate sales, thrift stores.. though the latter venues require an often heroic level of luck.

Internet auction sites.

Proceed with caution. In my experience buying this stuff on eBay, I can comfortably report that most of the sellers I bought from did not understand the equipment they sold. It is usually the sellers notion that they have a chance for a better sale if the buyer is under the impression that the device works. There is, of course, a huge kernal of truth in that; however the manner in which the uneducated (in archaic electronics) goes about testing this stuff can easily damage the unit. To draw an automotive analogy, you wouldn't pull a car out of a field in which it has sat for the last 20 years, gas it up and take it right out on the freeway without changing the oil and giving the brakes a thorough examination, would you?
Unless that AC cable is frayed or missing, the typical uninformed seller will plug it right into the wall and turn it on to allow them to gleefully report in the item description that "it lights up, no further testing done.". I address this in more detail in the burn in section.

Condition. I think there may actually be books written on the highly subjective topic. I know there are easily as many scales of condition as there are languages, and I've seen some instances where the stated condition had no perceptable basis in reality. Instead of delving into a tedious analysis of condition and rendering that into a common scoring system, I just want to touch upon the most general of states, which can be affirmed unless the seller is an outright liar or idiot.

• Non Functional. Kind of self explanatory, that. It could be an easy fix, but it's advisable to buy it as parts and enclosure, and nothing more.

• Untested. Huge ball of wax. In true terms, untested is not a bad thing at all, because proper testing often requires the sort of care, attention and know how that you're only going to find with devoted hobbiests or shops that specialize in this sort of thing. Usually "untested" will fall into two categories:
  

  • Unable to test; the power cord's been cut off, has a foreign connector, requires unavailable voltages or the power cord is visibly frayed and a hazard to plug in.

  • Unwilling to test; this isn't so cut and dry. There are many reasons I've seen stated for an untested condition, falling in a range of validity. Open a dialog with the seller if you need to bolster your confidence any, but do keep in mind that untested may mean broken, and in certain instances broken is permanent.

• Tested. This one is the killer. Unless the seller knows what they are doing, this one pans out like this:
  We plugged it in and the light comes on, unable to test further.
  I have also seen instances where the seller claims to have brought the unit up on a variac and it seems to operate fine, but may need calibration. I think anyone can tell the difference here. Do keep in mind that often these things have passed through several peoples hands recently before winding up at auction, so unless the cord is cut it's probably been dry fired once or twice already. Figuring you'll need to change the electrolytic caps anyway makes this less of a crime, but it's really nice to be able to bring the device up to full operating voltage and make sure everything else is in a useful or service able condition before dropping additional time and money into a recap.

• Works. Unless I'm led to understand that "works" means this device has been in recent use by the seller themselves, or has undergone restoration, I tend to lump this in the "tested" category.

My biggest tactic in inquiring about a given device is history. The farther back in the life span of the piece of equipment the seller can relay, the better.
Do keep in mind that these things are old. They have maintenance needs, and unless you're willing to get your hands dirty and work on them, they will eventually become either a money pit or non-functional regardless of today's condition.

Pictures can tell a lot about an item, unfortunately decent photography seems rare in the sellers circle. Below is a section of a front panel shot of a device that speaks volumes of its condition.

As one can expect without even seeing a gutshot, the transformer laminates on this thing are quite rusty. Rust on a transformer is a bad thing, it will function at impaired efficiency (since the laminates are more or less shorted to one another) which will increase heat, which in turn decreases lifespan.

I like to scrutinize equipment coming from areas of notorious humidity (like the southern US) a bit closer than something coming from say, Arizona. But the fact of the matter is that substandard storage conditions are substandard no matter where the item is coming from. Examination of condition culminates in a question of whether or not something is within your capacity to contend with. Obviously, this is a question only you can answer.

Shipping. Vacuum tube equipment is robust and delicate in a different manner than what we are used to today. Drop one on your foot to see what I mean. They are typically pretty heavy, which translates directly into rather high shipping costs. I've spent over double on shipping than on the device itself more than once or twice. So it comes as a huge letdown when you receive a 50 pound box that contains a broken unit and some rolled up newspaper.

Having someone you don't know ship you something heavy and fragile will always be largely a matter of luck and trust. My experience has been decent with only a few exceptions, typically my biggest complaint is packaging peanuts, which will break apart and find the deepest crevices within the device to hide, perchance to melt.

Your best bet is to respectfully open a dialog with the shipper beforehand and voice your concerns. This doesn't always work, as I've sent a nice and tactful email prior to packaging, been reassured that they know what they're doing, only to open a compromised single ply box with a laughable amount of padding a couple weeks later; fortunately the occupant of this shipping mistake survived intact, any scratch and dent picked up in route being concealed under 50 years of wear and tear.