Until you can find a replacement PSU or work out how to fix your existing one, I think you should look for a simple swap.

The Apple II FAQ (easily found on the net) describes the power requirements for the II family. There are loads of small PSUs that meet the requirement (and which will comfortably fit in a II), but you need some wiring experience before trying it. As an easier alternative, use a PSU from a pre-ATX PC externally -- you'll still need some wiring experience but it is a quick hack.