If you go to SUBARU GLOBAL and look at the Utility Accessories you'll see what's holding the floor mats. It looks like the clips are part of a bar that connects under the seat (where everyone can see a hole and don't know what it's for). I have a 2009 WRX, which is the same for 2008; don't know if it's good for others, but I guess it's for newer models.
I called Subaru America and, although the lady didn't think this could be added to inventory, she would look into it. I told her that the part was "missing" from american models and that Subaru at Japan should know about it. Maybe they can send us the part number or tell us where to get it.
In the meantime, has anyone found or can ask somewhere about this floormat clip-bar?