That earpiece works best (a cheaper deal is
here) , but you can do pretty well using an old style telephone tube (or just an earpiece from it) -
http://sci-toys.com/scitoys/scitoys/radio/radio.html . You can harvest enough enamel wire from any transformer or electrical motor. The coil core could be anything round of any diameter, but not conductive. The razor blades can be blued in bulk with propane torch. Electrical contacts - nails or paper clips. The most important thing is to provide a very long (high) antenna wire and a massive grounding point. Also you may want to make the pilot version of the radio with your particular components in order to tune it up beforehand (primarily - the amount of wire on the coil to catch the closest, or most powerful radio station in your region).
Many alternative ideas here:
http://bizarrelabs.com/foxhole.htmIf I would teach scouts, I'd provide them with a big enough broken AC/DC adapter and a telephone tube (the last component can be hard to find these days, though). No razor blade. You can find a diode or transistor almost in any electronics (the AC rectifier and LED works as an ordinary diode too).