Well, here is a list of a few places that may have the material you are searching for. Unfortunately, I can't tell you about the quality of their products or the prices.
The good thing is they are all located quite near to each other.
Jer: Hortaleza, 72
Jeco: Hortaleza, 82 and Pelayo, 8
Plaka Técnica: Colón, 11
La Riva Papelería Técnica: Plaza de San Ildefonso
Bellas Artes Pontes: Abada, 2
The best tips from your favourite hostal in Madrid.
Hostal Chelo at http://www.chelo.com