Just wanted to share my recent experience of MÁSmovil SIM cards being unavailable on the whole island of Gran Canaria. I checked half a dozen Cepsa gas stations, several Correos post offices in larger cities and all Telecor shops on the island and none had SIM cards in stock. Only two Telecor shops had some months earlier, while the others mostly even never heard of MÁSmovil.
So better purchase it online and let it be delivered to your Hotel.
I finally ended with a Vodafone SIM with the "Bonos por Tiempo Semenal" pack giving unlimited traffic for 7 days for € 19 (throttled to 64 KBit/s after exceeding 1GB). Unfortunately there are two issues for non-Spanish customers with Vodafone: On the one hand you can't recharge your credit from an ATM with foreign debit cards and on the other hand they don't accept foreign credit cards for online recharges at
http://tienda.vodafone.es/recargas, which would give a 30% bonus for each recharge.
Anyway compared to countries like the US and France (the axis of wireless evil) Spain has absolutely great and widely available prepaid internet offers!