The Mercado de Motores is held the second weekend every month at the museo de ferrocarril (railroad museum), about a 10 minute walk from my apartment (metro Delicias). This flea market is held inside the museum as well as the surrounding outdoor area. You can find a plethora of vintage clothing, art, and amazingly yummy food and live music.. and admission is free! Check out their flyer!
I spent a recent Saturday afternoon perusing the different vintage clothing stands and taking in that nostalgic smell of outdoor barbecues, enjoying the sunshine and listening to the live music playing on stage nearby. I even found me a big, comfy, durable jean jacket for 10 euros!