Hi,
You can find good tailors at hai yin fabric market. It is in da sha tou. There you will find all the fabrics you need, accessories and many good tailors.You don't need to buy big quantity there, they can make one piece if you need. It usually takes 3-7 days to make it. The price could be 250-500rmb for a qipao, but this is a rough guess.

If you need more information, feel free to contact me.

Lee-May