perhaps it popped off while driving, when there stolen, USUALLY you end up with none left. as in all 4 gone.
I can help you with new ones however, i bought my rims off someone, and 2 caps were missing. so i went to my local Import Car/Tuning Shop to buy 2 more.

but you can only but centercaps by the set (x4). so i did. it costed me $75 for 4 round white pieces of plastic for my ICW Racing Rims. so i suggest just go to your local Import Shop, and have them order you some.

unless the wheels are discontinued....which if so, it will be EXTREMELY hard to locate them.

good luck!