I am afraid I have no experience of rentals in the Costa Blanca but in the Costa del Sol the rental market seems to be saturated with properties and too few renters.
All of the selling agents that I have spoken to habitually vastly over exagerate the rental potential to secure the sale of the property.
For a long term tenant I would try the local estate agents, but for holiday rentals, I have found the only solution is to do it yourself. Agencies, from my experience and the experience of my friends, produce very few bookings.
At the top of this forum page there is a ‘search’ option. Try using this to find the subject of interest and read what others have discussed previously.
Good luck, David