Try here as an example, it is the most popular source for everything secondhand, but you will need to find a translator in a bar somewhere to call the owner, September is a good time because they are winter rentals for the next 7 months or so. Particulares are private owners, generally one month deposit up front etc.
MIL ANUNCIOS.COM - Alquiler de pisos en Torremolinos. Alquilar pisos en Torremolinos entre particulares.

thanks am grateful am looking for a long term rental as against simply for the winter. best wishes

I made a typo it should hare read there not they. There is a glut in the winter period but friends of ours have found normal long term flats with Milanuncios on several occasions as they progressively moved closer to the sea.
It is essential to speak Spanish , although you may have some luck using email and google translate etc. We have found a flat around Taragona using Trovit.es in this way, although we did speak to the owner in Spanglish/Spaitaliano.
M