I buy all of my books online. I actually do most of my shopping online. There are always people in the news saying that online shopping is taking away from physical retailers. But until they offer the same prices in stores, I'll keep buying online.
I know that many overseas people use Amazon, which I think is great, but sadly it sometimes costs lots to ship to Australia.
My FAVOURITE website for buying books is bookfinder.com
This website shows price of the book INCLUDING shipping to your country!! It searches lots of different book sites like Amazon, AbeBooks and Book Depository, etc.
My little trick is to look at Amazon to read the reviews and summary of the book. If you're not 100% sure about buying the book, I suggest you order it from you Library. If you're a member of the library, you can request books from other libraries. This is called an inter-library loan, and I use it all the time! Just ask your local library!
Western Australia has a great system, where you can request a book from any public library in Western Australia. Gosh, I love libraries.
(I just bought another book online for only $10 including shipping, so I'm pretty happy right now. Yay books!)