Is your Inspiron 660 a Desktop, or the 660s Compact Version?
If it's the Desktop Version, the Dell Inspiron 660 Desktop Mini Tower can use most generic standard ATX power supply units, with either a 24-pin or 20+4-pin main motherboard power connector, with or without the on/off switch.
You should be able to buy a compatible power supply from either a local or online computer store.
Note: You require a power supply with four SATA power connectors.
Check out the Corsair/Antec brands of power supplies.
The CORSAIR Builder Series CX500 V2 500W ATX12V v2.3, as shown here, should be fine:
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