First of all, are you looking for another stock one to fit, or are you gonna go aftermarket? I would seriously recommend aftermarket, because it has much better sound than a stock system and much more flexibility if you want to add a sub or other stereo equipment down the road. Installation is a breeze if you know how to strip and splice wires, or even buying a harness.
Of course, the stock one would be easier to take out and put in a new one.... but what's the fun in that