I ordered the small glass GM tube from Electronics Goldmine (along with 2,000 T1 green LEDs that I will split with a sudent). A small tube like this can be powered by a transformer and 555 timer.
However, I wonder if there's a best frequency for the transformer and tube. Rather that adjust a pot on a 555, I'll use a PICAXE-08M to make the square ware. That gives the circuit a greater range of frequencies. Also, having some intelligence in the circuit will permit the tube to be shut down upon detection of a cosmic ray. Perhaps that will decrease its dead time. If that's not possible, then the PICAXE can produce a single pulse at detection and ignore the other pulses until after the tube's dead time. That will ensure a flight computer (which will monitor only the PICAXE and not the GM tube) will see a single pulse per detection (rather than the 5 or 6 it sees with the kit I've been testing).