This useful post from Footie gives a number of suppliers Patsy.
SLA suppliers
You can't just buy the complete battery for your bike though. Inside the case are three 12 volt batteries wired together, so you'll need to open up the case or find someone to do that for you so those batteries can be replaced.
Then the connections should be noted so the new ones can be reconnected properly.
They come in various sizes, so you'll need to take the three dimensions of a battery in millimetres to be able to order the correct type.
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