I think the GM dealers are one of the few places that can get blank lock bodies. They have kits of tumbler parts that they use to populate the bodies to match the other locks in the car.
It is posible to get locks at parts stores and get them rekeyed... you may want to try a local lock smith and see if they can make one the same as GM dealers do. It may be cheaper than a parts store lock plus the rekey fee.
It is posible to DIY the lock but it can be quite ticky if you've never done it. It's hard to explain. It's more time consuming than dificult as long as you pay attention to detail.
On a side note... If the trunk lock has quit... also consider replacing the drivers side lock. That one takes the most wear of all them and probably isn't in that great a shape. Pasenger's side lock gets hardly any wear but can corrode up bad.
If you are doing one, you may as well check all them. It's probably cheaper than rekeying them one at a time.
Be alert. The world needs more lerts...
The Ogre's Fiero Cave (It's also at the top of every forum page...)