A good source for long-term rental accommodation is the classified section of the local weekly english press http://WWW.SURINENGLISH.COM – Some of the adds are by agents who will generally charge the equivalent of 1 months rent in commission. Some agents charge commission from the owner so this can be avoided or else try to secure a rental direct from an owner. If you know anyone who reads spanish then try http://www.segundamano.com where the spanish advertise property.
Marbella is generally the more expensive area for rentals – I pay 650 euros a month for a 2 bed 1 bath in Fuengirola on a small complex with pool and kids playground although it can be hard to find anything at that price. Look on forums similar to this one for people who have bought investment property and are now struggling to pay their mortgage. They may be happy to negotiate a good long-term rental rate to help cover their costs.