By "monitor," I assume you mean the computer itself. Additionally, with a 2008 iMac, you're in the wrong forum as it's undoubtedly Intel and not PowerPC. (EDIT: Either this post got moved, or I thought I was in the PPC forum. Oh, well.
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But in any case, you're probably in luck: while it's slightly decorated to conceal the plug, all iMacs take a "standard" IEC C13 cable (which plug into the standard C14 socket found in virtually all desktop computer power supplies, so if you see a cable labled something like "C13/C14," it's probably also good). You can buy one almost anywhere, or use one from an old PC (or monitor--virtually all of them also use this connector) you might happen to have.
But if you want an original Apple cable, Apple doesn't sell them online, and a cursory Web search seems to indicate that Apple phone support says they don't, either. Not sure about physical Apple stores, but knowing this other information, I wouldn't hold your breath. If you're afraid of eBay, try Amazon--there are several ones available, some of which claim to be OEM (i.e., Apple). But again, any standard cable will work as long as you don't mind the fact that it will lack the circular plate near the end of the cable to make it look pretty when it's plugged in.
