Step 2: Construction
The speaker was glued to the top of the case, with 3 holes drilled in the case for the sound to escape. A cabinet handle was attached to the case for easy carrying.
To operate the Geiger counter, turn on the switch (connects power from the 9V battery) and press the pushbutton quickly about 10 times to build up the high voltage. The high voltage will last about 2 minutes, then you will have to press the pushbutton a few times again. With the small tube I used, about 5 counts per minute are detected with normal background radiation. If you use a larger, more sensitive tube, you will get more counts.
These are very brief instructions, any questions please ask. Unfortunately I can't help with where to find a Geiger tube, and there seems to be a worldwide shortage right now. You can find them on ebay, but see the comments below, some types are not very sensitive so you might be disappointed. This link is interesting, it describes making a Geiger tube from a 35mm film container:
However, the air-filled tube requires >3000V.
From what I have been able to learn for example here , and in the einstlab link above, the special quenching gas in Geiger tubes serves only to reduce the 'dead time' between clicks, and allow faster count rates. When filled with air, a Geiger tube should still work but with count rates only up to about 300 counts/second instead of 10,000 counts/second. But hey, 300 counts/second is quite a lot and plenty good enough for do-it-yourselfers who are not likely to come across any radioactive sources strong enough to exceed that. So go for it, just take a metal tube and suspend a wire in the middle and it should work. I'd use something much smaller diameter than a 35 mm film case though, to avoid very high voltages like 3000V. An aluminum tube about the size of a drinking straw should be just about perfect (aluminum so the radiation gets through easily). With my simple high voltage generator, it sounds like something MacGyver could do in his kitchen! If only I had time to try it out... new baby in the house... need sleeeep...