Yeah, I am sorry to say there is just one bulb BD makes for the Supernova. It is a bulb designed for 3 Volts. Nominally, bulbs are specified at a lower voltage, like 2.8 or something, but it is a 3 volt bulb. The other BD headlamps run on 4.5 Volt bulbs. So, you want bi-pin bulbs that are for 2 cell flashlights.
Avalibility might become a problem since the light is discontiued. I bought quite a few spare bulbs for the Energizer Double barrel lights when Wal-Mart cleared them out.
What you can do is solder a simple bi-pin bulb onto the base of a BD bulb. As long as you hold onto the burned out bulbs, you could recyle the base. Without the base, most bi-pin bulbs will float in the socket. With the avalibility of bi-pin 3 volt bulbs, you really have some options. Just don't get carried away and melt the reflector (as I have done). Keep the current below an amp and you should be okay.
The PT Yukon bulb (a 4.5 volt headlamp) works much better than any of the BD headlamp bulbs in all other BD headlamps (except the Supernova). It is well worth the trouble of soldering a base on it. The bulb is avalible at REI.