They were discontinued in 2006 so spares availability went into decline. The batteries were discontinued almost two years ago and are no longer available.
All you can do now is to use another battery and arrange connections from it to the battery platform connectors. The battery can be mounted on the carrier or rigged into the usual space it it fits there.
I'd recommend you use a 24/26 volt lithium battery. One such supplier is
BMS battery, from China but uaually cheaper than UK sources, despite having to pay duty and VAT on entry. Many of theirs include a charger, which is important since they can be expensive on their own.
Before buying though, are you sure yours is a working bike? Quite a few of these by that age have failed motor units, and no spares are available.
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