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Chops is a trusted home cook.
In NYC? Any Jewish bakery - too many to list. Challah for Rosh Hashana is round instead of braided to signify a new year and sometimes the bread is made sweeter to signify a sweet new year. A round challah is often dipped in honey for the same reason - a sweet new year during this high holiday. Should be very easy to find; Fairway and Zabar's...
You might also find one with raisins or other dried fruit to add more sweetness.
Cynthia is a trusted source on Bread/Baking.
slx, good to know. Challah is one of my favorite breads, and I was only taught the braided and multi-braided versions. I love your story of the sweeter, round ones.
amysarah is a trusted home cook.
Unfortunately, there aren't nearly as many authentic Jewish bakeries in New York as there once were. As mentioned, Zabar's would probably have one, but the only really old school authentic Jewish bakery that pops into my head in Manhattan is Moishe's in the East Village (2nd Ave.) I'm sure they'd have it.
There are probably also some left in Brooklyn, Queens or the Bronx too (maybe in Borough Park, Flatbush, etc.)
Suzanne is a trusted source on General Cooking.
I have heard that there are some really great Jewish bakeries in Borough Park and Williamsburg in Brooklyn. I will see if I can get a name and post it.
Outer boroughs like Brooklyn is a hike, if you have no car and relying on the crappy weekend subway schedule. Plenty in the heart of NYC...
If you're looking to buy in NYC, private email me for a place.
June is a trusted source on General Cooking.
FWIW, those bakeries would likely be closed on Friday afternoon and Saturday. Shabbat.
Lisanne is a trusted home cook.
Fairway sells several brands, also has their own brand which is very good. Even better is Fisher Brothers' challah (on 72 bet'n B'way and West End). Their challah is yummy. Just the right sweetness. Also excellent is Butterflake Bakery, in Teaneck. You don't have to go to Teaneck to get it: the Kosher Marketplace on Broadway @ 90th stocks it at the end of the week, certainly by Friday morning (and possibly Thursday night). Fisher brothers and Butterflake are the best, in my opinion. Other than my own :)
Rosh Hashanah challah is not necessarily sweeter than at any other time--it's just... rounder.
I forgot to add, as ChefJune indicated, kosher stores like Fisher's and Kosher Marketplace close early Friday and are closed Saturdays.