Elizabeth and James' gold polished knuckle ring retails for $225 at Saks Fifth Avenue.
Urban Outfitter's smooth knuckle ring is a great option if you're on a bit of a budget. This ring retails for $24 and you can click on the photo below if you're interested in purchasing.
*Update - 4/12/11* - Forever 21 has a similar knuckle ring on their site (this post is originally from August last year so the UO version may no longer be available) for $3.80 and also comes in silver. Click here if you're interested in purchasing!