As K7HTH said just use a ATX PSU they are easy to find and get the adapter from Amigakit. The only issue you may have is the +5v wire on ATX PSU units is to small for the curent draw that the Amiga needs so by dubbling up the +5v wires from the ATX PSU this will give you what you need (all +5v wires go back to the same place inside the PSU). On the old amiga 4000 PSU this is the thicker yellow wire. If you still have the A4000 PSU and its dead cut off the connector wires close to the PSU and then physically marret the wires to the proper wires on the ATX PSU leads this will give you the best contact.
