The power supply for your PC is an OEM PSU made by Delta. Sony part #146870923.
Sony, Dell, HP/Compaq amongst others use proprietary parts where possible (notebooks are even worse) so if a part fails, you have to buy from them at outrageous cost. They'll tell you there's other reasons but we all know why they do it.
Even if you could get a larger rated PSU from Sony it would cost a fortune. Better spending the money re-casing the machine and enjoying a much wider choice and better value PSU. Here's a link to someone who did this before - using exactly the same motherboard (Asus P4SD-VL). Also be careful moving the board into the new case - you can't get a replacement from Asus at it's OEM again.