I have cycled through many, many resources over the years, but these are my current favorites.
TIP: I often recommend that students buy a used copy of the teacher’s edition of any textbook they’re using in school. That way you can see if you grasp the material as you go along rather than having to wait for the teacher to correct it. Also, the teacher editions often have helpful tips in the margins. For example, the teacher edition of a math book might point out typical problem areas for students and how to address them.
Generally, Abe Books has the best prices for used textbooks, including the teacher editions.