While I have not specifically stayed in Cavalaire s/ Mer, I have rented apartments along the Mediterranean coast of France. What I have generally found is:
1. Vacation rentals usually are month to month. You may have difficulty splitting a few days out of the following month (the first 3 days in July for example).
2. You´ll save a great deal of money dealing with the apartment owner directly and avoiding agencies. This of course means you must have a relatively good command of the language as these negotiations are generally conducted on the phone.
3. It is very difficult to try to help someone unless he specifies a budget in euros.
4. The best place to find independent owns with apartments to rent is to visit the location and look for rental signs (appartement a louer)
5. The next best place to find apartments is through the local Office de Tourisme. They usually keep a complete listing of available properties.
http://www.cavalaire-sur-mer.fr/se_loger.php
6. Many owners book their apartments with families who visit yearly so the earlier you start looking the more likely you´ll find what you want. The most desirable properties book at least a year in advance.
Good luck and persistence pays dividends.