Kosher Asian stuff in New York City

I do a lot of Asian cooking and have been kosher-ish in the past, so I'm somewhat familiar. A lot of products are completely vegetarian and so should have no problem being kosher, but often do not have a hechsher because they did not consider a Jewish market; this is especially true of foods produced in Asia.

fish sauce

Doesn't exist. Red Boat produced a kosher fish sauce for a bit, but they stopped because the increase in sales didn't offset the cost of certification. If you email them, they'll tell you that they haven't changed any processes since they got the certification.

dark soy sauce

rice wine vinegar

Ohsawa and Eden Foods make a variety of Japanese foods, most of which have a hechsher. You can find them in fancy grocery stores and online. I don't remember if either has proper dark soy; if not, the best substitute is a combination of regular soy sauce and tamari, possibly with a little sugar added.

oyster sauce (I know this is a longshot, but maybe there is some kind of kosher equivalent?)

There are any number of vegan oyster sauces, usually based on mushrooms. I can't think of any that are kosher, though. Vegan oyster sauce recipes are pretty easy to make; the hardest and most expensive part are the dried shiitakes, but there are several dried shiitake mushrooms with a hechsher (I know that Frontier Coop does some). Unfortunately, most rabbis are of the opinion that dried mushrooms should be hechshered because of the risk of contamination with other foods during production. You can always dry your own (fresh mushrooms are always kosher), but that's a pain in the ass and it's important that the mushrooms be dried for these recipes.

red chili oil

Honestly, just make your own. It's super easy.

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