I destroyed a motherboard here at work from a PC, solely for the purpose of nicking it's battery for the A4000.
You can use any PC battery I think, aslong as you put a diode in there to stop the Amiga trying to charge it when the power is on. If you try to force power through a lithium "coin" battery then they can explode.
Just gotta stop that and you'll be sorted.
Look for LithBatt.lha on Aminet, it'll explain it a whole lot better than I have :-)