First, avoid UPC codes.

They're expensive + cumbersome + Amazon no longer uses them... or rather... even if you have a UPC code, you'll require replacing these with the Amazon AISN number assigned you.

https://www.gs1us.org - UPC Mafia, where you purchase UPC codes.

More important is a high throughput multicolor printer (to print ad-hoc labels) + a barcode reader device, to ensure your barcodes are accurate (can be read by Amazon barcode scanners).

Ebay. You can get 1000 for around $15.

I'm not an expert on this, but a friend who is an Amazon seller recommended getting a GS1 tag and going from there. The website provides clear and easy-to-follow instructions.
https://www.gs1us.org/

If you have questions on building your organization, product and business strategies, shoot me a message!

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.