Lavalle, Lavalle, Lavalle!!
The fabric stores basically start at Junín y Lavalle and go NorthWest (not that anyone knows what that means) towards Pueyrredon.
From Lavalle to Corrientes on Junin you have about 5 stores or more. From Junín to Uriburu on Lavalle you have about 6 to 8. (There's another one on Tucuman ~2057 b/n Ayacucho & Junín next to the boutique hotel across the street from a puteria which is just down from a BUTTS Video... which is across the street from a pre-school, but forget about that one.)
From Lavalle to Corrientes on Uriburu, you have a bunch more.
If you're eating a jamón y queso sandwich while shopping, try not to bump into any men wearing black suits, a white shirt, no tie, but with a black hat. They have an aversion to jamón y queso sandwiches.
A Recoleta location mentioned in the area above:
There is a store on the corner of Uruguay & Arenales (I believe) where you can get really nice fabrics at probably 2 to 4 times what you'll get in the "jamón y queso sandwich-free zone".