It's not in the book, but...
In some cultures, here in the real world, there are lots of superstitious folks who will collect locks of hair and even items like nail clippings because they believe these items grant power over the person they came from.
Now imagine what it must be like in the D&D world where this is actually true.
For example, in Curse of Strahd, the sidebar on p. 29 explicitly says:
A secondary goal [of Strahd's spies] might be to acquire some physical object — a possession, an article of clothing, or even some part of a character’s body such as a lock of hair — that Strahd can use to improve the efficacy of his scrying spell.
I would also imagine there'd be a pretty active (and potentially lucrative) black market in personal items and actual bits of detritus from a person's body.
I would also think, especially among underworld/criminal/spy types, there might even be a requirement that you give up a personal item, lock of hair, drop of blood, or what have you, as insurance when you join certain types of organizations.
Basically, it's a form of insurance that your underworld boss could have someone spy on you whenever he wanted to (with -10 to your saving throw via the scrying spell)
So giving up a drop of blood to your underworld boss might just be business as usual. And if not, I can see underworld types arranging to acquire these things, even from non-minions, as a form of insurance or leverage.
That is pure speculation on my part. But the more I think about this, the more the I may incorporate this sort of trade into my campaign.
