center hubcaps?

Now that snows on the way, the steelies are going back on. I thought it would look better if i had some hubcaps like the police crown vic's have them. I wouldn't know how to mount them though, how do those things stay on and where can i find them?

These are 15X7 by the way. Would the ones on Ebay be suffice? I know someone has to of done this...