Slapped this on today. 'Murrica.
The sticker was actually free. All you have to do is send a self-addressed stamped envelope to a sign company called USA Traffic Signs and they'll send you an assortment of flag decals. You'll get three waving flags (the one in the picture above), plus three large standard flags and three smaller standard flags. I don't know how durable/UV-resistant they are, but you can't beat free. I heard they are decent quality, though. Here are the details: Long Lasting American Flag Decals
I've always had a flag on every vehicle I've driven since I started driving. I tried a static cling on the WK2, but the rear window tint is too dark. I had a 3M Reflexite flag on my Patriot, and that was my favorite flag decal because it was nice to have something reflective on the driver's side tailgate: Reflective American Flag Decal by Reflexite
(I wanted to do the same with the WK2 but the 4x4 and Laredo badges are in the way).