Yeah that is a bummer.
Expensive part. Around $460.00 GM List price. According to alternative GM sources like the Mitchells it is called a Backup Lamp Assembly. Also, it has the running/park lights and the license plate holder and license plate lights. Your best bet would be a salvage yard. Also, try Ed Morad at his Ebay store. He specializes in used GM "W" body car parts like for the Regal.
Very easy to remove and replace. Lift the trunk and on the back of the lid you'll see the fasteners. Just remove the plastic wing nuts or standard nuts depending on which design you have. Several of them. Once you remove those it will detach and you will see the wiring harness(s) and various connector to the lamps.