Most textbooks will be available at the co-op bookshop. Closer to the start of semester, you will be able to see what books are suggested for what subjects at this website:
The Co-op Online Bookshop - textbooks, books, course notes, study guides, software
However, I would advise you to wait until semester begins to get these texts - because sometimes they aren't required. It's a good idea to wait and see what your lecturer says is necessary.
You will also probably need to buy course readers - which are photocopied collections of book chapters and articles. You can find which of these are available through the "Students Notes Onlne" section of myuni - which you won't be able to access until you enrol. These course readers usually aren't available until ~1 week before semester begins, and sometimes even later.