I put this page together so I had quick access to a high quality, automatic barcode generator for my book covers. Just type in your ISBN below, and the price (if you want your barcode to have one), and you'll get a zipped file of your high resolution barcode instantly in two formats - EPS and PDF.
The program will automatically convert 10-digit ISBN's to their 13-digit equivalents.
PS: Check your barcodes! These should work everywhere, but before you print a thousand copies of your book, please test the barcode to make sure it works. There are many smartphone apps that read barcodes; you can even scan the barcode right from your computer screen - if the book is already for sale it should show you comparative prices. If it's not yet for sale, it may pull up the information from Bowker, but this will depend on the app you're using.