where do BBQ restaurants in Seoul buy their brisket?

I'm beginning to research the topic of brisket in Seoul. I think I want to make barbecue brisket sandwiches next month for a goodbye party: two staffers are leaving on March 22 and 29, respectively, and I'm already committed to making gyros (this time with homemade flatbread, thanks to a recipe I got from my buddy Charles), but since one of the guys who's leaving shares my love of brisket, I thought that I might as well buy a hunk, cook it up, and turn it into sandwich meat. It won't need to be a gigantic slab of brisket; I need only enough to feed seven people, maximum. Question is: where to find a legit brisket?

This old website has some info about meat in Korea, especially meat-related terminology. I don't know how legit the Korean translations are, but the page appears educational at first glance. As for buying meat: I've heard about the Majang Meat Market, which may or may not have what I'm looking for. Brisket has to be sold somewhere locally, though, right? I mean, the American-style BBQ restos in the area can't be importing all their beef from the States or from Down Under, so there must be a local supplier.

Anyway, that's March. This week, I'm making my famous Middle Eastern chicken for a coworker's birthday (and also Just Because), and since another coworker requested cheese fondue, I'll be prepping that as well. This ought to be interesting. Again, this will be a small event compared to some of the massive ones I've done in the past: just us R&Ders. That's good, especially given how labor-intensive the chicken is.