Hello and welcome to the forum!
This indeed is a problem for may visitors to Tunisia asnd finding reputable estate/rental agents can be a pain.
We have added a new property rentals section to the website
http://properties.tunisia.com which has an increasing number of properties to rent in Tunisia.
You can also try other leading sites where owners let their properties unfortunately I would treat renting a property from a Tunisian estate agent in the same manner as renting from the owner (prefer it actually) as many estate agents are one man bands in Tunisia so do not hand over all the money and do you homework before coming to the property.
Regards,
Kris