The hub carries the weight of the car. The lugs are designed to hold the wheel to the hub, but not to support the weight of the car. That is why so many people end up popping lugs off and losing wheels in the street.
Many aftermarket wheels will have a hub opening that is large enough for the biggest application of the wheel. The consumer (you) driving a car with a slightly smaller hub needs to fill that gap with a hubcentric ring so the hub holds the weight of the car - not the lugs.
I don't really know where to find them as I've always purchased wheels that are a direct fit for my cars.
