Shopped garment district new york city last summer: only thing wrong was so many choices, it took too long. My favorite store was a junky looking place named Material World; Some fine fabrics, some junk, but oh, the prices. It is listed in the Manhatten phone book. Threads Feb/Mar 2001 had an article “Shopping for Fabric in New York”.
Also there is a discussion line about this article at number 2572.l Garment district is well known on maps, tourist guides, etc. I found the local people very helpful and friendly. Shopping in New York was my lifetime dream trip–happy hunting.