The classic source for model interrogatories is the form book, especially Bender’s Forms of Discovery: Interrogatories, which has helped me out many times. If that doesn’t work, you can search for a well drafted interrogatory filed with a state or Federal court in one of Westlaw’s new interrogatories databases (INTERROG-FED for Federal filings; INTERROG-STATE for state filings; INTERROG-ALL for both). I added the Westlaw databases to the Interrogatories entry, following the discussion of helpful form books.
p.s. If you work someplace with a litigation practice, you can also search your Document Management System and hopefully get a well drafted in-house model interrogatory for free.