The iMac's 27 inch screen is a totally different monster from normal 27 inch monitors.
The normal ones are all 1080P, a low resolution for the size, the iMac on the other-hand, has a 2560*1440 IPS screen, which has almost twice the amount of pixels. Unless you buy a Dell U2711 or the HP Pro monitors that costs ~$1000, you wont be getting the same thing.
You'd be better off going to a place that sells TVs.