Hello,
It is not so difficult finding an accomodation in Reunion island. You have masses of adds on the local newspapers ( especially the Quotidien which publishes special pages on Saturdays for house renting or buying and the "Gratuit", which is a free adds paper coming out on Mondays).
You can also see the add boards in the main supermarkets or local boutiques although the adds there are not always up to date.
The best bet remains "le bon coin" or some other internet sites for adds on line.
you can also contact agencies. Most of them will offer an alert on your email box when they have an accomodation suiting your criteriae.
Prices generally range from 350 euros/month for a small studio to 1200 euros for a 4 or 5 bedroom house. With variations of course. Count 800 euros for a 3 bedroom standard house.The North and the west coast are the most expensive areas wheras the East part of the island or the far South offer normally much more reasonnable prices.
The problem with agencies , apart from the agency fees ( about one month rent is that they ask for a 3 months notice to leave the house or the flat. As a result, they often offer you places to rent that you can't actually occupy before three months later.
In the meantime, you can sometimes find people asking for a "gardiennage" of their house while they go on holidays...some agencies specialize in registering some potential "gardiens".
As for the best time of year to find a house to rent, try July when there is a lot of movement of civil servants going back to metropolitan France.
I hope that answers some of your questions
