Ferreteria Hach? in SD has a specialized fabric and curtain outlet, part of their 'muebles y instalables' division. I don't know what it is called or where it is exactly based though.
Bartolomeo
Galleria Dumit on calle del sol and also try Multicentro salvador on bartelome colon in santiago. They have huge selection of fabrics for all kinds of needs in huge industrial sizes and quantity.