Depending on your case (Chenming, Coolermaster, Lian-Li, Silverstone, or ThermalTake) they have options under Case Parts-OEM. But if not the window kits are usually the way to go now (many have pre-cut fan holes too). It's easier than cutting the plexi yourself (caused much swearing when it cracked on me). Installing it requires little more than a Dremel, or a Jig-Saw and drill with some time (I'm old school, so I like the Jig-Saw, it has a cleaner cut).
@Box read the previous posts before commenting please.