The fastest way is going to Factory as a scav and looking for scav bots with a respiratory mask, killing them and taking masks out.
Note, that killing other Scavs while playing as a Scav badly influences your reputation with Fence.

The easiest way to find medical bloodsets is to look for them at the medicine spawn sites. For example, look for them on Reserve in the medical unit (white elephant building), on the ground floors of the Health resort, or in medical bags.

Also, a Medical bloodset can be crafted in the hideout at the Medstation level 2. 

Hideout module	Components		Produced item(s)
Medstation 2 level	Components	Tube 8 000 ₽ For 1 item at the flea market 8 000 ₽ For 1 items at the flea market Syringe x2 26 111 ₽ For 1 item at the flea market 52 222 ₽ For 2 items at the flea market 60 222 ₽		Produced item(s)	Bloodset 0 h 27 min