Wheel bearing dust cover thing...the thing that pokes through the center cap of the rim.

 

I don't know where you can get a Mazda-specific one (other than the Mazda dealer), but NAPA sells generic dust caps. Measure the inside diameter of the hub opening, and then check here...

http://www.napaonline.com/masterpage...rsal&VehCode=N

Edit: If link doesn't work, go to NAPA on line, enter vehicle info, and do a part search for "dust cap".

 

Oh, I never even knew those things existed.