I am always looking for the cheapest place to find the best books. For years, Amazon has been the go to site because of their wide selection and free shipping on orders over $25. However, lately I have been hooked on Westminister Bookstore. They have all the best books and cheaper than Amazon (sometimes a lot cheaper). They also have a very helpful structure for finding books.
For example, if you want to see the best commentaries on Galatians, where would you go? You cannot just go to Amazon and type in "Galatians." You will get a whole list of "bunk" commentaries. However, at Westminister Bookstore, you can go to "commentaries" and see a list of every Biblical book. Click on Galatians and you will find the best of the best. And, you can trust the theology to be solid.
Take a look for yourself.