The original, short, USB keyboards had power buttons, but because of USB limitations, they did not have the versatility of the old ADB keyboards. Unlike the old |> key you could not use the USB power button to restart the computer, you had to push the power button on the system until it shutdown, or hit the reset switch on the B&W G3s and the Graphite G4s. You can often find the old USB keyboards for <$5 at your local thrift store, or you could look on either craigslist or ebay as some people upgraded to the newer keyboards for the full size and media buttons.
When the newer black keyboards came out, there was a company making power buttons to attach to the USB on your keyboard. I don't know if they are still around, but it might be worth checking out. Also, on the newer (not newest) keyboards, using the Eject key with either Control, Option, or Command yields a different result, there are more details on Apple.com.