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Contacting the original builder is sometimes an option.
The key to finding a second source for a battery is identifying what connector is on your bike. The dimension between the pins, the pin diameter, and a picture could help experts to identify it. Vernier or dial calipers are handy for this.
If the battery has a custom mount, and you wish to maintain that, then the original vendor is the only source likely. If you could hang a bottle or shark battery from your frame, then more options are available.
What country you live in will guide who are the most reliable generic battery vendors. In the US I've found a couple of pushers of ****, and one good vendor who was expensive but worth it.