I buy them from Bundle of Holding, when they're on offer. https://bundleofholding.com
I recently gave away a huge amount of my physical books. So much money I spent on them and most of them were now out of date and not things I'd ever get a chance to play in person.
I prefer having physical books to pdfs when I'm tabletopping, I've just not tabletopped in years. And having a tablet that can contain all my pdfs and then some, is so much more preferable to the space than two large boxes of books takes up.