Just northeast of where 26th July Street meets the Cornice (in Cairo proper - so on the east side of the Nile) is Wikalit al-Balah.
Wikalit al-Balah (also known as Souq Bulaq) is a bustling market area that sells a wide range of stuff at low prices, including clothing, handcrafts, second hand goods and even spare car parts! But the big draw - and what the area is famous for - is the fabric, especially the cotton. Select your colour, pattern and exact material, and buy it by the metre. Although prices are reasonable, you will be expected to bargain hard.
Wikalit al-Balah is the best place to go in Cairo if you want to buy the material to have some clothes made. Most stalls are open 6 days a week, closed on Sundays.