I need a couple of "plugs, knockout plugs, etc." for my wheel pants.  I had a buddy lose one while doing some work and now I need to replace it.  It is the standard round piece (about an inch or so in diameter) that snaps into the hole into the pant.  They are for removing when you air up the tires (I'm sure you all knew that :p).  Anyway, if someone could please clue me into exactly what they are referred to as, and where I might order some (Lowe's or Home Depot would be nice).  Thanks.

Steve