If you already have an iMac or a computer with MiniDisplayPort, this may be of interest: http://support.apple.com/kb/HT3924
It seems you need to be able to boot the iMac. I'm not sure how you would turn it on otherwise. It isn't like a monitor.
The 21.5" iMacs do not support display input, so you would need a working 21.5" iMac and something like ScreenRecycler.
see > ScreenRecycler
Another option is to use another working 27" iMac that supports Displayinput or an Apple Display with your 21.5" iMac.
If you really have your heart set on identical monitors, then you would be far better off with a Mac Pro or Mac Mini and a matching set of Apple, LG, Samsung or other name brand monitors..