There are many math tutorials on the web (for free) that you can use to help you study for the quantitative section. Just because they're not GMAT study guides in the explicit sense does not mean that their content is any less valid.
Moreover, you can learn a lot by participating in this forum (its free as well); plenty of questions and plenty of smart people answering them. Alternatively, you can download the 1000 SC/CR/RC/PS/DS to help you in your studies(they're free) - these sets are readily available on this forum.
Also, I recently posted 6 gmat exams that you can download. Remember, you have to log in to see the attachment.