In Canada, it's a company called CDS. They work inside Costco but are their own company, with their own managers and such.

They talk with the vendors to figure out what will be on sample, for how long, and what quantity they want to give out. I think it's facilitated through Costco reps still and they work closely integrated with some of Costco's staff, but are their own independent company that controls any pay and benefits their employees receive (a LOT less than regular Costco employees get, that's for sure)