Welcome, I am not a fashion kind of a guy but what I know is… Harajuku and Shibuya are considered as fashion center for young people, I will emphasize young people. Fame of Harajuku regarding fashion started by young starting designers. On back streets of Harajuku young starting designers who did not have much money got together and rented small ground level apartments facing a street. Usually 3 to 5 designers rented an apartment and started to sell their own items. Magazines for girls in teen and twenties started to pick up on that and TV drama was made based on that concept. That was in ‘70s which made Harajuku known for fashion all over Japan. Now Harajuku has famous designer’s stores on the main street, but if you go to a side street you can still find small stores by unknown designers.
Why Harajuku? Harajuku used to (talking about 1950s and 1960s) have a lot of small apartments renting to university students. An apartment was nothing to write home about, but rents were cheap and owners did not have problem renting to young people without job. Which worked out for young starting designers who did not have jobs.
Shibuya has large concentration of clothing stores and considered as fashion district for young people. Right front of Shibuya station there is 109 Building, there is a big sign on the top of the building says “109” so difficult to miss. 109 is one of famous location for clothing shops for young women. Pretty much can bet any teen girls in anywhere in Japan knows about 109 at Shibuya.
About fabrics, I had to look up little bit but since fashion for young people are concentrated in Harajuku and Shibuya, many fabric stores are located at Shibuya. However, looks as most fabric stores are small and located at side streets.
All above is about fashion, clothing, and fabric for western (European) style clothing. In Japan when you talk about fabrics there is another kind, it is fabrics which is used to make Japanese kimono. Those fabric stores for kimono is called Orimono Tonya (織物問屋) and historically and even today it is concentrated in Nihonbashi (日本橋) near Ginza. There are several stores which sell fabric for kimono in Nihonbashi.
I looked up and could not find anything about Sendagaya and fashion. May be if you can provide source where you got an information about Sendagaya then can get more information.