You can purchase the barcodes from the sites here:
barcodes4u.com
buyabarcode.co.uk
speedybarcodes.com
snapupc.com

And this guide might come in handy:
https://www.gs1us.org/upcs-barcodes-prefixes/get-started-guide

Would tak precaution if you are even considering to purchased upc barcodes from ebay as there might be recycled barcodes being sold and might lead to your products being identified as fraudulent and shutting down your amazon seller account.