A GH6 on paper will make 6v, 1.8w at 15 mph. In reality, the outputs vary a bit because of the age of the parts and small variations in manufacturing. The Reflectalite guide says that the original bulbs are no longer made, but in theory you could substitute another bulb if can properly balance with the rating on the tail light bulb. I used to do this by buying a variety of bulbs from a bulb warehouse online (a few dollars would get you a half dozen or so bulbs to mess with) and then experimenting until I got the most I could out of the Dynohub. An old school hardware store might also have bulbs for old flashlights, and I guess what you'd do is buy several and experiment until you get something decent. That being said, old school flashlight bulbs are disappearing fast now that even small flash lights can be had we really strong LEDs in them. A "quick and dirty" source might be a bulb from a Miller lamp set, but the bulb likely will have low output because it is rated for the 6v 3w Miller system (the GH6 will not supply enough power to put much light into the Miller bulb).
Over the past few years, I've become a convert to using LED bulbs in these lamps to improve the light output even more. If you're going to be riding the bike much, I'd recommend the Reflectalite LED conversion kit. It's an LED head lamp bulb, an LED tail lamp bulb, and a current limiter to prevent burn out at high speed.