Where to find a separate charger for Braun toothbrush?

I can no longer charge my Braun toothbrush model 4713, but as the battery still buzzes for a few seconds
when switched on, I think the fault may be in the charger or its lead - type 4728 - which fails to show a light on the brush. I have checked the plug/ socket
and fuse which are OK. I can't find any indication of
the charger unit being sold separately from the brush.
Can anyone help please? (I do not feel technically
competent enough to try changing the battery!).