Salamanca is the area with the most concentrated local fashion offerings, especially on Calles Claudio Coello, Lagasca, and the first few blocks of Serrano. You'll find a good mix of mainstream designers, small-scale exclusive boutiques, and multibrand stores. Most mainstream designer stores are on Calle de Serrano.
The top stores for non-Spanish fashions are mostly scattered along Ortega y Gasset, between Nuñez de Balboa and Serrano, but if you want the more exclusive local brands, head to the smaller designer shops unfolding along Calles Claudio Coello, Jorge Juan, and Lagasca. Start on Calle Jorge Juan and its alleys, and then move north along Claudio Coello and Lagasca toward the core of the Salamanca district.