People keep saying you can only buy the Simyo card online but here:
https://www.fring.com/forums/showthread.php?t=14489
Someone mentions:
"Check
Telefonía móvil sin ataduras, tarifas bajas, móviles libres e internet móvil, they use the Orange Network and have great phone and internet rates. You can order a SIM or buy it in one of the authorised stores (not many). You will find a list with google-maps links on their site"
And indeed on their website there appears to be a link to a map of retailers:
Liberadores simyoC/ La Moraña 7 - Google Maps