Fabric lovers around the world know that Japan has some incredible textile designers. You can find all of them and more at Nippori Textile Town, home to 84 shops that go on for blocks selling fabrics, notions and sewing services.
From the JR north gate at Nippori Station, take a right and then a down escalator to the street. You'll see a sign to the right pointing to Nippori Chuo Dori. When you reach one of the first shops, look around for free Fabric Town maps in Japanese and English. (The Nippori Sen-i Gai site linked below also has a PDF.)
If you have limited time, head straight to the main Tomato shop on the left. There are several shops called Tomato, but the one labeled 52 on the neighborhood map is the most impressive. The first of four floors has wall-to-wall cotton prints, including well-known Japanese lines for cheap. They sell by the whole meter and only accept cash.