The PA3/SPA4 needs 19 volts power to run and to power the fan on the heat sink. Any old laptop adapter at 19 volts should do.
The small plug (laptop plug) will be cut off, so don’t worry about the size. The wires are either attached to the terminal block on the PA3, or you will solder a new plug on the cable and a corresponding socket on the metal enclosure for easy connection/disconnection.
Price: $5 - $15, available anywhere, here is one from eBay
Note: This power supply is to power the PA3 itself, not the plasma tube. The power supply from step 4 earlier is to power the plasma tube.