It can be cheaper to buy books from abroad. To my knowledge the simplest way is to buy from https://www.bookdepository.com. This bookshop has got most of the books, and all the prices are freight inclusive.
The bookshop http://www.amazon.co.uk is the biggest. They also have used books. But their freight and handling charges can be extensive.
Bookdeository offers to pay using paypal. It is easy to open an account with paypal, and it is safe. You only need to present your credit card once – when you open your account. https://www.paypal.com/no/webapps/mpp/home
If you think this is difficult, you can go to your local bookshop and they will order the book for you.