I used to squirrel away paperbacks all year from sources mentioned by the O.P. Would start each long trip with a grocery sack full of books, donate them as finished along the way.
Perhaps you could shop online for used books and have them sent to you along the way?
Or, gasp, bite the technology bullet and get a Kindle or other tablet? Maybe a refurbished one to save money? I thought I would never let go of physical books for my recreational reading, but my kids gave me an iPad and I have never looked back. I check out e-library books or buy Kindle books to read on the iPad with Kindle app.
