I personally don’t like the taste of oyster sauce (Gul sauce, 굴소스) on its own. It tastes too artificial for me. However it makes some good dishes (like Mushrooms on rice) when used with other ingredients.
This is a picture of the oyster sauce I am using.
It is about 2000 won (US $2.10) for 255g, you can get premium oyster sauce for double the cost as well.
Its color is dark brown, it is quite a thick liquid and very salty on its own.
I didn’t know which brand to get, I just bought this, because it has the longest expiry date than other brands. Though apparently this (Lee Kum Kee) is one of the most popular oyster brands in the world. (I googled a bit)