Generic neons are... well for one thing nearly extinct, not to say unavailable, but production is very down AFAIK. But availability aside, what is available (new or old) tend not to be the cleanest of vacuum tubes. That is, due to impurities in the gas fill, electrodes, surface oxides and other contaminants, exact metal composition, maybe even some things about the glass -- the glow pattern, voltages, lifetime, and how everything varies during that lifetime, all all pretty dubious for circuit use.
Your starting point would be -- first, I mean you can buy regular tubes and test them, see if it works, at all, or for how long, maybe measure, characterize and sort them, separate out the most wobbly ones. You'll find audio noise (flicker, sizzling, popping, etc.), unstable light output or pattern (glow coverage of the electrode(s)), variable strike and hold voltage, etc. vary from part to part, with operating conditions (Ia), and sometimes you get lucky and one is good as ever, and many are just kind of crap, when you need any kind of precision anyways.
The next best thing then, would be premium quality ones -- I doubt anyone is producing these anymore (but I guess a lot of searching around might be worthwhile?), but other than that, a look through an old e.g. GE glow lamp manual ought to produce ideas (many of the above posted links/references are excellent as well), and you can look up datasheets for voltage regulator, oscillator/switching/computing types, etc., and see if you can find like a carton or whatever of them floating around somewhere for cheap. AFAIK, these are largely your bog standard NE-2's or whatever, just made with better materials and process control, so that the properties are stable, noise is low, and lifetime is acceptable. There may also be types with getter included -- though they might not be leaded bulb types, but more proper pin-base tubes as such. (I *think* you don't want voltage regulator tubes, 0G3/85A2/etc., as they're specified for fairly narrow current levels, for which the regulation voltage is nominal, and they may drift at currents outside that range -- may still be usable but be careful with pulse discharge sorts of circuits.)
Note, my assumption: a change in operating condition, may exacerbate chemical changes within the tube, that weren't controlled for in design and manufacture. NE-2's are basically made to glow and that's it; and even that, they weren't exactly great at(!). So, using them for VR, switching, etc. applications is very pot-luck, or dubious, or finicky and short-lived, and that's why special types exist (and even those weren't very good at it).
Tim
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