I would definitely try JobFluent ( http://JobFluent.com ). They are a job board completely focused on startup jobs and they are based in Spain. Plus, they organize networking and recruiting events. read more
Europe Language Jobs is a multilingual job board, the job offers published are not exclusively in Spain but you can find many in this country! And what's best is that all offers are looking for candadites with language skills (so Spanish won't be compulsory in many cases). read more