For indicator lamps and other tiny ones, fing anything that fits the socket.. If it burns out, try a higher voltage/wattage bulb.. experiment..
If you can't find a bulb that fits the socket but can find bulbs with other base types, you might be able to cut your bulb socket off it's wires and attach the available base type onto the wires. Radio Shack sells both bulbs and their sockets. This solution partly depends on how the bulb is stuck into the speedometer.
Lately i have been using LEDs for tiny indicator bulbs. These are rugged and can be used on any voltage. The only thing you need add is a resistor appropriate to the voltage to limit current to the LED. White, high output LEDs are nowavailable and i wouldn't hesitate to use one for my speedo. (any basic electronics booklet will tell you how to calculate an appropriate LED resistor and how to solder, etc..)
Most bikes have magnetos and no voltage regulator. The faster the engine spins the magneto, the more voltage is delivered. As a result, many 6 volt systems often put out perhaps 15 volts at top speed.. and a 12 volt may deliver 25 volts. This is why bulbs burn out prematurely.
The fix is to keep going up in voltage and watts. A high voltage /high wattage bulb in a 6 volt bike is not likely to burn.. However, it's light output might be dim at idle. There is no cure for this other than building and using a voltage regulator (which is beyond most people).
As far as finding exact replacements for headlamps, i posted my solution in this thread. Finding the dual-beam with the correct adjustment tabs is difficult and expensive. My solution limits me to High-beam only. The "low beam" position is now "OFF".
One other thing i had to do was wire the tail light to the high beam circuit. The tail light now lights only when the High-beam is selected.