I thought Mobis' online mall would be a good place to start looking: http://mall.mobis.co.kr/mainindex.jsp There were some spoilers listed, but not for the XG. Then again, I don't read Korean very well and I could have easily missed it. Or maybe Mobis doesn't sell them online.
Next I looked at the XG site on Hyundai Motors Korean site. No spoiler again in any of the official pics, so I don't think it's an OEM option. Man, I got mad looking at all the cool options for the KDM XG. Rear adjustable and heated seats, navigation, refrigeration in the center console for beverages!!! The list goes on and on...if you want to see what we're all missing check out the e-catalog for the XG on that site. Okay, off my soapbox.
You said you've seen them in Korea so I'll assume you get there from time to time. Maybe visiting a Mobis location there would get you hooked up or at least they can point you in the right direction.
Otherwise, I didn't see any of the usual suspects here in the US (e.g. Kspec) offering a rear spoiler and of course the Hyundai dealerships here are no help...the range of OEM accessories for the XG here are just dismal.
This is a long way to say I didn't find anything, but I was curious and gave it a shot.

uzzled: