|A 9v battery could power that thing. |
As long as it uses a standard ATX motherboard, nearly any power supply will work with it. A machine like that would pull around 45w from the wall, and the smallest ATX PSU I've seen was 200w. As OtheHill mentioned, small computer shops usually have tons of old power supplies in storage.
If you come across a Dell power supply, don't use it! Back in the P2-P3 days, Dell used their own version of ATX. Dell PSUs looked, felt, and smelled like standard ATX components, but they changed a few pins on the motherboard power connector. Plug a Dell PSU into a standard ATX motherboard, and you'll see smoke.
mATX Beast | LGA2011 i7-3930K @ 4.2GHz | 16GB quad-channel DDR-1866 | GTX 670
240GB SSD+2TB HDD | Asus Rampage IV Gene | Seasonic X560 | Silverstone TJ08-E