Evans is £36 at the moment:
http://www.evanscycles.com/categories/a ... tAodVycAEg Two chargers, not sure which suits your lights or if there is actually any difference between them.
In terms of finding anything cheaper you need to look at the info on the charger.
Matching the input should be simple enough, matching the output could be trickier (
xV,
ymA). If you can find something with the same output details it will also need the same socket fitting or you'll need to purchase/make a suitable adapter.
There'll be tarmac over, the white cliffs of Dover ...