They are called hubcentric rings. Stock rims are made to fit around the hub of the car. Aftermarket wheels typically come with a larger bore size so they can fit multiple car types. The ring centers the wheel around the hub and also allows the wheel to correctly carry the weight of the car.
To find the size you need, go here ->
http://adaptitusa.com/referencebook.htm
You can find them online by searching google, or call a local Discount Tire.