Hello,
Probably the two best sites for dissertations are ProQuest for national coverage and the Center for Research Libraries for foreign content.
Those two sites are among the list you find by searching at the "A to Z Database List" at the main library site, library.fiu.edu. When you run the search there you are taken to this page,
http://libguides.fiu.edu/az.php?q=dissertation
Not all the listed databases supply full text sources, though. And web searches (or specific university sites) will also turn up results that sometimes you did not find from the list.
Please use the Ask A Librarian link for follow up if you have any further questions.