There are several sites where you can do UPC web scraping for products
walgreens.com UPC Web Scraping
It is easy to extract UPC field for walgreens.com site, look at the image:
walmart.com UPC Web Scraping
For Walmart, you can extract the MPN field & Dimensions also!
bhphotovideo UPC Web Scraping
B&H has UPC inside HTML / json of a product page.
6pm.com UPC Web Scraping
6pm has UPC inside HTML / json.
Site where you can scrape products UPC