Bikes can indeed be hired from Roda FM but they don't have many, and I would be surprised if they have any left for mid-August.
They can also be hired in Los Alcazares - 2 places in Balneario Centre and there is also one on the rear road behind the shops and bars at the Oasis Arches. Sorry - don't have names or numbers to hand.
They are not cheap - €10 per day although I think that RodaFM charge €50 per week, and they will not be "decent" bikes. They will almost certainly be run of the mill, mid-range ones.