The black cat is a curious creature, long linked in the mundane world as a creature of evil and mischief.  Though having nothing to do with evil, it is true, that they are close in tune with the energies of magic ley lines.
 
Commonly found in the studies of those that practice the art, they are drawn by those same energies and usually make themselves at home when they are content. They provide comfort to the practitioner of the art, help to keep the study, lab, and home, free of pests and if lucky, they evolve self-awareness. It just so happens that my lab has two of them. Luna and Eclipse are perfectly happy to spend their days hunting mice and making themselves a permanent fixture on my windowsill.
 
Such creatures in tune with magic, are sensitive to those that roam the other side, or those who use magic to hide from sight and even can sense when there is impending danger. Luckily for me, my apprentices can find plenty of their hair in my study. Thus, providing ample opportunity for my apprentices to collect them and bring them to you. Corked and sealed in a vial, to preserve its potency. Sufficient for all magical potions, magical studies, and spell components.