Archive of Our Own (AO3) is another great place. It has a large collection of fanfictions, and it's well - organized. You'll be able to find various interpretations of the relationship between Sakura and Sai, from friendship - focused to romantic ones.
Some Naruto - centered forums also have user - generated fanfiction. For example, NarutoForums.com might have some good sakura haruno and sai fanfiction. You may need to do a bit more digging there, but it can be a great source for unique and less - known stories.