If you are looking for a place to rent in Spain but aren’t here yet, it can be difficult to get a sense of the places you are viewing online, not to mention getting an owner to pay attention to your email. Most people here want to meet you in person and won’t even consider someone who is not here yet. Plus, places rent very quickly!
Your best bet is to find a room through Homestay, Airbnb, or some other homesharing site and get your bearings.
Then you can take your time looking on Idealista.com, Habitaclia.com or Casamona.com and visit places in person. I would stay away from placement agencies as they take a huge commission (2-3 months rent) or even a monthly commission, and they don’t necessarily have exclusive apartments that you don’t see on the sites listed above. The exception would be business apartments that are solely rented to corporate customers.