if you don't find it online your best bet is to call or email the NA offices of TSW and tell them the story, more often than not they will send you one in the mail for free. Just say you are taking the car to a carmeet and really want to use the TSW annd rock their rims our but not if it's missing a centercap.
enkei has done this for a friend, varrstoen sent me one also in the mail...actually they sent me two

at TSW YMMV worth a try in any case.