BarCodeGen allows you to create your own ISBN barcode. ISBNs can be obtained by purchasing one, or a block, from your regional source, such as ISBN.org in the United States, www.isbn.neilsenbook.co.uk in the UK and other international agencies.
Once you have an ISBN you can type the number in with the keyboard. As you type, you get immediate feedback on the ISBN itself. If the barcode turns red, the barcode has an error (the checksum doesn't match). After you've typed in the full 13 digits, the resulting barcode should be black.
Once you have created your barcode, you can save it to your photo library (for later porting after sync'ing), or you can e-mail yourself a copy of the image.
BarCodeGen is on the App Store now, for iPhone 3Gs, iPhone 4 and iPod Touch devices.