Popular online real estate sites include:
Idealista - http://www.idealista.com/pagina/portada
Pisos - http://www.pisos.com/
ThinkSpain - http://www.thinkspain.com/spanishproperty/

You may also try using an agent, Agentes de Propiedad Inmobiliaria. Agents usually charge a fee equivalent of one month of rent. Agencies will have clients sign a contract that includes deposits and agency fees. Investigate an agency thoroughly before signing a contract. Beware of ads with contact numbers listed like: "93000000" with an asterix as these are often scam agencies. Look for an apartment rental agency that is a limited company - they have strict tax and booking-keeping to maintain for the Spanish authorities. Businesses with these accreditation are marked with the letters S.L. or S.A. after their business name.

Best of luck!