In our day we are blessed with the ability to buy almost any book online, and often cheaply. Online shops make it possible to buy older books that are not stocked in bookstores anymore, and even books that are no longer in print. This is great, since many good books are often older books.There are many bookstores available and you can choose the best ones for your location. I find ABEbooks has the best selection at the best price, but this will depend on your location. I have tried to add the ISBN number for most books to make it easier to search for them.
Buying used books is great. You can get a second hand book online for a fraction of its cost new. Books do not lose their value by being read, if anything, they become even more beautiful. If you are embarrassed to get someone else's child a used book as a gift, then get them 2 or 3 used books as a gift, and that will make up for it. Even better.
Finally, I want to say something about Audiobooks. Audiobooks are a great way for children and young people to enjoy a book. Especially if the book is written using difficult language, it can be easier to understand if you listen to it being read. You can also listen to audiobooks in the car together during commute times. Unfortunately I find that the most popular audiobook subscription sites have more bad books than good books available. One solution to this is to buy audio cds if you have a player for them. Naxos Audiobooks for example have recorded many of the great books. Librovox also has recordings of thousands of older books. They have many of the classics, and they are free to use. Of late they even have a well-functioning app with a simple user interface. They are certainly worth looking at. Additionally, the one audiobook subscription site that I have found that seems to have a more Christian-friendly collection of books is called: audiobooks.com. I would not include all of their books on my list here but they have a notably better selection than other known providers (I am not affiliated with them).
Happy Reading!