Replacing Wheel Center Caps

I've got a 2004 X-type 3.0. Over the past 6 months, I've lost 3 of the 17" Cayman wheel center caps. Today, I was washing the wheels, and two of the caps fell off while I was drying them off. It seems that the plastic clips on the caps break off and only the minute amount of axle grease that leaks past the seals then holds them on. I'm tired of making the 20+ mile trip through LA traffic to get to the only dealer (Rusnak, Pasadena, CA) close enough to get them replaced under warranty.

Has anyone else had this same problem with breaking clips, and if so how did you solve it? Also, is there a site on the internet (outside of Ebay, I'm tired of bidding then waiting to see if I won) where I can get replacements reasonably?

Thanks,
Jacab