I would suggest visiting Mahmoud's establishment aka Al-Wikala in the Khan al Khalili market as soon as possible after your arrival. He has a wide choice of beautiful, high-quality costumes in all sizes and an excellent seamstress who can make adjustments within a few days to a week. You may require a custom costume if there is none close to your size and/or style preference. Mahmoud is a pleasure to deal with, honest and courteous. One thing; his prices are fixed, so no bartering as with other Egyptian merchants
I have another excellent resource in Mohammed Ali Street, but the info is at my office and I'll post it for you later.
Khalida