Where can I buy the power cable for my iMac?

have you tried googling for answers? I'm sure multiple vendors sell them.

The AC power cord for most desktop Macs, including iMacs, is a standard IEC cord available at any electronics store, many variety and hardware stores.

It is used as the standard power cord on countless electronic gadgets and appliances. They are readily available and there is nothing special about the ones for Macs except they are prettier.

Ones rated for 13 Amps and up are superior.

Have you asked your local Apple store if they can order one for you?

Otherwise, the Intel iMacs use a generic computer power cable ... ebay has lots of them for iMacs, and even Home Depot carries the cords (although in black and without the circular piece to seal the back).

Found some here:

http://www.dvwarehouse.com/iMac-Power-Cords-c-2376_5414.html