Hi Hien,
Yes, you will still get access to the associated online resources if you purchase an e-book elsewhere. However, the time of access will be 6 months/e-book. In order to receive the online resources, you'll need to email a receipt of your iTunes purchase to
books@manhattangmat.com. Let us know if you have any further questions!
Best,
Sam Fleming
Student Services Associate
Manhattan GMAT