Are you talking spacers or adapters? There is a difference. Spacers simply slide on the wheel stud between the wheel and hub and use the same wheel studs to fasten the wheels on sandwiching the spacer. I am NOT a fan of these at all due to the undue stress it puts on the actual wheel studs.
Adapters actually bolt onto the original hub using the original wheel stud, the good ones will have the same conical seat as the wheel which center the adapter on the studs so they are concentric with hub. They then have another set of wheel studs that the wheel actually bolts onto. If you are looking to space your wheels outboard a small amount this is what I recommend using.
Mike.