Sano Outlet Mall feels pretty much like an American Outlet Mall. In fact I think this company also manages the outlet mall in Las Vegas (which is amazingly good BTW). You can find plenty of international brands here. I've heard people complain its hard to get large sizes here.
Sorry for the awful photo. Its taken before we started to take photos seriously.
You can get there easily from the highway or you can get there by train too. Details are at the site listed above.
The outlet is expanding and will open a new section in July 2008. Also I hear Nasushiobara will be getting its own outlet in July near the Shinkansen station.
You can combine this trip with Ashikaga pretty well.