You want one with the same or higher wattage, the same style motherboard connectors and same form factor so it fits in the case. You can get a microATX power supply at 145W or more at any computer store or do a search on froogle as I did and found one for $17.
See the power output and select the one you need however these prices are on the high side for an old power supply but considering that these power supplies are pretty small. You might not find any better prices for them.