Replying again to clarify:

I do get that anyone who's slayed a vamp or whatever might be too broad for our purposes, and that sticking to those with a "Chosen One" label might be a little more plausible. I'm just saying that if our gangs could get ahold of proper Slayer blood to me that starts raising a host of questions and problems that we'd need to solve in order to let everybody contribute. And the easiest way to deal with those questions and problems is to say that they can't get ahold of proper Slayer blood.