Apologies, I had thought you had already been using the site run by the Eveche in Tunis. So here is the link:http://www.geneanum.com/tunisie/bases.html
The site is in French, but even if you are not familiar with French, I think it is easy enough to follow...
So, you can input the name(s) of your family and see what comes up. I would recommend that if you enter the full name and get no info, then just enter the last name and then scroll through the lists that appear. You see, often the spellings may not be correct. For my family (Alessi and Drago) the spelling of first names varied a bit year to year.
The reference no. you need to reference when submitting a request for copies of certificates is the "repertoire" number on the right of the page.
Instructions for ordering are on the site, but I think they only take Euros. So it was the same for me (being Switzerland and currency is the Swiss Franc) had to send Euros.