Well we'll never really know, but I took it as a sign that the Founders probably used yet another race they control back in the Gamma Quadrant to produce it for them. Given what we know of the Dominion pretty much dominating (pun intended) whatever trade, commerce, and peoples they want in the quadrant. Also since we know the Vorta are bred to basically be the Founders right-hand-men/administrative lackies, it's not likely something the Vorta produce themselves.
I'm sure it's not, but I have a hard time believing the Founders would entrust something so important to another group given their mistrust of solids. After all, if the group were bold enough to corner the market on the drug, they
would have the loyalty of the Jem'Hadar, not the Founders. Why take the risk? Weyoun himself said that the Founders' ability to control the Jem'Hadar is overstated.