Switching power supply can be picked up from most any source of coin-op amusements distributor. Their prices range from $15 to $35, depending on what they offer. Another source is computer power supplies, which generally supply the same voltages (however, some do not provide enough amps on some of the voltages). Most standard arcade switching power supplies output +5VDC, +12VDC, and -5VDC (and sometimes -12VDC also).
There are a few links out there, but most analog Williams power supply problems can be fixed by replacing connectors, resoldering and sanding the header pins, replacing the 2N3055, the LM723, or the bridge rectifier. If you've tried all these, and it's still a no-go, I don't know what to tell you.Typical Failures of Early Wiliams Games
Give up? Bob Roberts sells replacement power supply kits that you can pop right into your Williams game.Replacement Williams Power supplies (near the bottom)