You should be able to find taillight bulbs in any LBS that carries dynamo systems. If not, you can order them from Reflectalite. They're in the UK, but they ship to the US and I've never had a problem with an order. The bulb you want is the GV601 Screw-cap 6V 0.1A 0.6W

http://www.reflectalite.com/vacuumpage.html