thai flat rice noodles
You're best off going to these noodle/tofu houses. I only know of two in Manhattan. One is:
The second one has moved recently. It's on Grand between Mott and Elizabeth (North side of the street). I don't know the name, but the women who run it used to be on Grand and Bowery.
There's one in Brooklyn on 8th Ave in the high 50s. Sorry don't remember the exact streets. Not a store, but some women selling them in front of a store.
Haven't come across one in Queens yet but I'm sure they are out there somewhere.
Bangkok Center Grocers or Udom Grocery will not carry them. Occasionally you may find them at some Chinese grocery stores, but I wouldn't recommend it as they will be refrigerated. Once the noodles are refrigerated, they stiffen up and don't behave properly. You should prepare your dish the day you purchase the noodles.