You're looking for the active ingredient pentobarbital. Nowadays almost exclusively used as a veterinary drug. Normally between 10-15 grams of pentobarbital is used to ctb.
The veterinary formulation comes in liquid bottles. Sometimes a color is added to make it stand out (commonly blue or red), it otherwise is a colorless liquid. The concentration of the veterinary formulations can also differ. 6.3 grams per 100 ml is common in some countries (and is most commonly acquired by members), whereas other formulations can be 20, 40 or 50 grams per 100ml.
Another route has been to acquire pentobarbital powder from China. In which case they ship about 25 grams from what I understand.
Recently it has become much more difficult to get N from what I've read here.