Hello, I'm from Madrid.It's difficult to find a hostel near the venue because the stadium is situated far from the city centre.The stadium is located in what it seems a huge, "far-from-civilization" field, so you should look for hostels in the city centre or north-east zone, look for "Puerta del Sol", "Gran Via" or "Avenida de America" zones, and then you can go to the stadium using the underground or the buses.The underground stop near the stadium is "Las Musas", you'll see that it's the last stop of the line number 7.Did you try this link?
http://www.hostelseurope.com/
Hope it helps! :P