I need to know where I can purchase a COMMERCIAL database of UPC codes
(lists manufacturer and product). I am not interested in open
source/free databases. I am curious as to where grocery and other
stores get them.

Hi team2o,

Businesses generally build their own databases of the products they
have for sale, but Gregg London does offer a UPC database for sale.

Gregg London's Product Load Services
The situation...
You just installed the latest Point of Sale (POS) System for your new
Convenience Store (C-Store) and now it's time to enter seven thousand
grocery items - by hand!
You need to take inventory on your latest acquisition - a Specialty
Foods / Spirits store - but you realize that there is no portable
technology present to do so.
The solution...
- Product Load Services
Pre-Built Product Files - containing U.P.C., Category, Manufacturer,
Brand, Description, Size, and Unit of Measure.
For further information, please contact me at 469-366-4278, or via E-Mail.
http://www.glondon.com/productload.html

Gregg London's UPC Databases
http://www.glondon.com/upcdatabase.html

UPC Database Downloads (free)
"Here you can find downloadable copies of the data in the UPC
Database. Each full download contains a file of Manufacturers by
manufacturer code, and a file of items by UPC. The files are ASCII
text, comma-separated values (CSV), without column headings. Text
fields are quoted as necessary. These files should easily be readable
by MS Access or Excel (if that's your thing), or Text::CSV (if Perl's
your thing). The CSV files are bundled together with tar(1) and
compressed with gzip(1)."
http://www.upcdatabase.com/

Additional Links of Interest:

UPCData.info
http://upcdata.info/

"This is the most complete list of bar code related web sites anywhere
on the net. You can search for a site by keyword or company. You can
look for all the sites by topic or you can browse the entire list."
http://www.barcode-1.net/pub/russadam/websource.html

BarCode1
http://www.adams1.com/pub/russadam/new.html

GEPIR 
http://www.gepir.org/

dmoz: Computers: Software: Bar Code  (233)
http://dmoz.org/Computers/Software/Bar_Code/

I hope this helps. If you have any questions, please post a
clarification request and wait for me to respond before closing/rating
my answer.

Thank you,
hummer

Google Search Terms Used: commercial barcodes database