Hola
I think Madrid would be your best option. There are two or three shops with outdoor equipment Check out the sports/outdoor section of the Corte Ingles or this shop: Ruebles de Las Montanas in El Rastro. They will tel you where the other shops are. Also there is a Decathelon in Madrid
http://www.decathlon.es and they definitely have ponchos - whether or not they have the one you are looking for you will need to enquire.
Decathlon
Sporting goods superstores
Address:
Parque Oeste, Sector 1 - Barrio Salamanca
28922 - Outside Madrid
Madrid
Nearest Bus or Train:
Tren: Cercanías C-7 (Commuter train)
Open Hours:
9a-10p M-Sa
Decathlon Inesis Castellana
Paseo de la Castellana, 110, 28046 Madrid, Spain
+34 915 61 65 55
Hours: 10:00-10:00 PM
Sunday Closed
Public transport: Nuevos Ministerios (196 m W) 6, 8, 10
also
Avenida Rafaela Ybarra S/N, M40 Salida 24. Autobús: 60 / 131, Madrid - 913 410 080