You can build your own using a shunt (1mv/A should
work fine), a 741 op amp, and an LED. The 741 and the LED are available from Radio Shack, but you'd probably have to buy the shunt from an electronic supply. Total cost of parts, including a case to put it in, should be around $20.
To do this, connect the shunt between the starter solenoid and the rest of the bike's electrical system (connecting it to the solenoid rather than the battery avoids running the starter current through it, although a 1mv/A shunt would probably be fine with starter current if you wanted to connect it directly to the positive battery terminal). Connect op amp pin 2 to the electrical system side of the shunt, pin 3 to the battery side of the shunt, pin 4 to ground, pin 7 to a switched power lead from the fuse box, and pin 6 to a wire running to the instrument cluster area. Mount the LED somewhere around the instruments, then connect one leg of it to ground and the other leg to the wire coming from pin 6 (the package will tell you which leg should be connected to ground).
If you want to use an incadescent panel lamp instead of the LED, use a small relay with a 12V coil. Connect one side of the coil to pin 6 of the 741 and the other side to ground. Run a wire from a switched power lead in the fuse box (the one that's connected to pin 7will work fine) through the relay contacts and up to the instrument cluster to power the panel lamp.
Basicallly all you're doing is building a comparator. When the battery side of the shunt is at higher voltage than the electrical system side (which is the case when the battery is discharging), the 741 output will be high and the LED will be on. When the battery side of the shunt is at lower voltage (which happenswhen the battery is charging), the 741 output will be low and the LED will be off.
If you're feeling adventurous you could try doing this without the shunt, instead connecting pins 2 and 3 to the positive battery terminal and the fuse box, respectively.I have no idea if you'd get enough voltage drop in the wires between the battery and the fuse box to make the circuit work reliably, although you certainly wouldn't do any damage by trying. This would bring the parts cost down under $10.00.