I assume you mean the common 6V 3W halogens that are common for generator systems that are not powering a taillight. An expensive solution would be to order direct from the UK at reflectalite.
Depending on whether your generator's output is DC or not, you can go with various LED conversions as well. I'm not that familiar with generator systems. Do they tend to be DC, or AC? Also, are they constant voltage, or constant current (what happens when you go too fast?)
FWIW, I have two 6V 2.4W halogen bulbs I'd gladly sell for $5 a piece, $8 for both.