Also known as Dongmen Lu Fabric Market, this is one of several cloth markets in Shanghai selling all kinds of fabrics and tailoring services and accessories.
For fabrics like silk and cashmere in particular you’ll want to check the quality carefully as stallholders are prone to exaggerate, and bargain the price of the fabric and the tailoring together for the best deals (but remember, you get what you pay for). Check the quality of the clothes on display on the stalls to get an idea of their work and pay attention to seams, lining and hems etc. It’s wise to take along an item to copy rather than get too creative here. 60% of the time you’ll need alterations so be sure to leave enough time for changes (if small, these can often be done within a couple of hours but it’s wise to allow a day or two before departure).