Ive seen Salmon in the chinese Fish markets for about $3 a lb.
Fish is not like Meat where it has to be Kashered, It just has to be cleaned and its the same cleaning from a Kosher Place or a Chinese fish market.
BTW according to that OU website its just doesnt ave to be Salmon, It can be other fish too as long as you see the Skin and can identify it. Flounder is actually pretty easy to indentity as long as you see the fish whole as its a Flat Fish and its eyes on the top of it rather than the sides (Flat Fish do not swim like you think, they swim on the bottom of the ocean sort of like a floppy Pizza)