I would use one of the laptop ones really. Your selection will be limited due to the base/arm bit but you can still get a few that work well I think. I don't know that anyone ever thought of iMac decals, maybe there is money in that.
You might have to make your own, but something like a Chicago skyline would be relatively easy to do.
Buy a piece of vinyl from a craft store or a hardware store and trace the picture onto it. Print out a picture at the correct size to use as a reference. Use an x-acto knife to cut it out.