Very little hope of finding ANY (you can read this as almost nil/zero/zilch/nada) kind of genuine and quality Korean antiques and even less so weapons.
Instead of wasting your time to find something, better focus on visiting their excellent national museum and the 4 palaces in Seoul.
You can buy some genuine, albeit newly made, traditional green Koryo celadon, some laquer-ware and maybe some Andong Hahoe wood masks or a pair of Korean wedding ducks. You may find all these in high end souvenir shops in Itaewon district. However, be aware that most of cheaper souvenirs sold in abundance as being Korean made, are in fact from China.
To get an idea of the types of sovenirs you may find, you can browse
www.korean-arts.com
They sell genuine, more or less traditional Korean souvenirs, but in my oppinion, they are your best shot.
Have a nice and enjoyable trip!
Marius
PS: I lived one year in Seoul.