They will have batteries at a salvage garage, but i doubt you'll really know what sort of condition they'll be in, although for the batteries you want to charge they'll probably be ideal for them.
There's shit loads on Ebay but obviously mainly collection only.
I would be inclined to either go to, or phone, a few local mechanics garages, tyre firms, Kwik Fits, etc, and see if they have any old ones laying about that you could have. Just tell them the truth what it's for and they might be glad to get rid of it for a small drink, or even free.