The best replacement for isbn is upc.

The best alternatives to isbn are rated as following:
★★★★★  upc
★★★  issn
★★★  ean
★★  isbn 13
★  dewey decimal system
★  sku
★  call number
★  asin
★  library congress number
★  isbn-13
★  library of congress
★  isbn
★  barcode
  lccn
  keywords
  isbn13
  dewey decimal
  upc number
  doi

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Speaking about isbn, one probably shouldn't forget about createspace.

Other search terms related to isbn and their ratings are:
★★★★★ createspace
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