There are many sites from which you can purchase books from, including AbeBooks, Book Depository, Alibris, BookOutlet, Kidsbooks, Walmart, Booktopia, Thrift Books, Books A Million, Amazon Books, Barnes & Noble, Better World Books, Biblio, BookMooch, Book People, Target, Costco, Daedalus Books, eBay, Powell's Books, The Strand, Hudson Book Sellers. 
If you cannot find the books there, you might want to consider downloading a digital copy from annas-archive.org instead.
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