Where can I get King Trumpet mushrooms in Manhattan?
Check your local Asian markets or well-stocked upscale groceries, e.g. H-Mart, Whole Foods, Fairway, Gracies Market, Eataly, etc. Call ahead to see if there in stock and/or carry.
I found them recently at Barzini's, a well-stocked bodega on Broadway at about West 91st.
I've seen them in Chinatown markets quite often. You could check the markets and supermarkets on Mott and Hester, or Bayard Street market. The advantage of getting them in Chinatown as opposed to other places, is that they will be way cheaper.
I just saw them at the Manhattan Fruit Exchange at the Chelsea Market