If you don't have access to what Rich suggested, what about use cases? The last position I was at, I had put in place functional specs, test plans and test cases to give them a foundation. I spent quite a bit of time with the users and worked with them to figure out positive use cases which then evolved into the more formal documents.
Well, as already has been told, in best case you write the test plan according to system requrements (business rules, use cases or whatever you call it) - in general according to some documents originated by product management or marketing and describing the functionality of the system.
Second best case - interveiews of product managers/marketing people. If they do not take the time to write the requirements, at least they know how things are supposed to work
Not-so-good case: Iterviews with developers. They know how it will work (which is not always how it is suppose to work)
And the worst case - accoering to ready product.