You can get -5V by connecting something like an
LM7905 3-pin regulator chip to the -12V rail. That's 7905, not 7805, which is the positive version. Also there are voltage regulator modules (DC-DC converters) that can put out -5V when connected to +5V or +12V.
I don't think many modern PSUs are rated more than 150W for the +3.3V and +5V combined, but old Antec TruePowers and SmartPowers had ratings like 180-220W combined, even the 300W units.