Velvett,
You can get Nam Pla (patis fish sauce) at almost any Korean Deli. The Viet Namese version is called Nuoc Nam, basicly the same stuff, the Thai vesion is a bit stronger. I think the joint is the Japanese stuff, ShottSuru, and that should be all over your neigborhood... your on the West Side, no? Just go to little Korea and snap some up.
If not, anyplace down in that little japan area in the forties just west of Grand Central will do... chinatown will have it, but that might be a bit of a haul. Postive Zabars and Gourmet Garage have it...
If you want to guarantee the total freshness, and see the best food/fish mega store: go to that wicked Japanese grocery store right on the river in Fort Lee.... can't remember the name of it, but it is TOTALLY worth the trip, the choice and quality of Japanese ingredients will amaze. Grade AA Toro? No problem. Shiso so fresh it snaps? Ginger Shoots? Fresh Konbu to make your own Dashi? How about a dozen or so different kinds/strengths of Bonito Flakes?
Too too cool.