They buy barreled bourbon.

From the article:

He sends his formula to large distillers, who send him what some know as moonshine and others call "white dog," a raw unrefined form of whiskey.
"The moment it arrives here it is bourbon," he said. "It is at least 51 percent corn (rye, wheat and barley are also there) and has been aged in barrels." But he acknowledges that the aging that takes place before he gets the raw whiskeyfrom the distillers is negligible, and does not add much to the six months he uses to age it