For fans of hardcopy: Where do you get books on the road?

Goodwill, Library, garage sales. They go in the campground bathrooms with a note on the they need a new home when I am done.
I have a bookcase that rides on one of the bunks when we travel so I usually do not need to buy books during the time we are on the road but I do anyway.
The books travel in two Rubbermaid containers that when empty sit under the folding table that sits next to the trailer. Table has a ground length fitted table cloth on. It took about ten minutes to make.