I found a box of 50 unidentified working ones at a nearby antique shop. I paid $10 for it.
Good luck. Since tubes aren't being used much these days, there isn't much of a supply of junkers.
If you want real tubes, one approach would be to contact electronic repair shops in your area -- preferably those which specialize in old equipment -- and tell them that you'd like any dead tubes they replace with newer ones. The only other good source I can think of would be to watch for tube electronics being trashed -- or to put out a call for those on Craigslist or Freecycle or something of that sort. Either way, you're only going to get a few at a time and the supply will be erratic. Offering a finder's fee might help encourage folks to make the effort to help you.
If you're willing to spend more money, I believe some tubes are still available.
Or... What most Steampunk fabricators have been using is light bulbs in tube-like shapes. These have the advantage of being a lot more commonly available. They don't really look right, but on the other hand they do glow, which the dead tubes probably no longer do. (On the other hand, they need higher voltage; tubes were mostly designed to run on low voltage.)
Tubes are higher voltage (something like 22 volts) for the heater element, small bulbs will usually work at 3 volts or so.
The local surplus store (Ax-man, Minnesota) has a box full.
If you get to know the local thrift store workers (or even volunteer there...), you could ask them to set aside old electronics so you can salvage from them. Most of the time they just pitch them.
Ham radio fests would have some-but they are going to be expensive.
You could 'fake' some with electronics or mechanics in small plastic or glass containers, depending on what effect you are looking for.