Over the past few years, I was on a quest to find a place to look up UPCs (as part of my job at Pandora). As you probably know, there are few reliable sources. This year, Sound Exchange released a new lookup service which can return UPCs and ISRCs.
You can do reverse lookups too. I discussed this new service with someone from SX. This data comes to them directly from the labels so it is pretty reliable. So far I have had good results. They report that more features are being developed. Check it out.