I wonder how they're getting it? You would think the list of carfentanyl manufacturers, suppliers and dispensers would be pretty small.
It is a synthetic opioid, an analog of fentanyl which has no clinical use because of its strength. As such, it hasn't hit the abuse radar until relatively recently, and isn't regulated like clinical drugs. It is, most notably, an animal tranquilizer for very large animals.
Clinically, because it is 10,000 times as potent as heroin, the chance of a lethal outcome with a very small dose renders it useless... however, mixing half a gram with a lot of manitol or other substance makes tens of thousands of doses of ersatz product with a huge street value. If that isn't done right, someone is going to die.
Since veterinary medicines are not regulated the same way, nor intended for human consumption (look at veterinary antibiotics, for instance), most veterinary medicine can be purchased with relatively few strings attached or hoops to jump through.
https://www.opiates.com/carfentanil-linked-heroin-overdoses/