Sony Playstation move controller, the thing with the glowy ball.
On its own in case you want a second one or already have the PSEye.
This is cheaper than all gaming shops and a lot of places online!
Also, Currys pricematch other shops so grab other accessories whilst you're there e.g. the gun holder is £12.99 at Argos.
Also see other deals on here for Asda and John Lewis.