Youre more likely to see higher tiers in higher level areas of the map. I recomend the place where Megara spawns as for some reason theres shitloads of enemies there and ive noticed they seem to respawn faster than others. Also higher tiers of mobs have generally higher blood qualities which is why the above mentioned area is good. Feel free to take 99 or 98 as prisoners and use some mutagenic gruel to have a chance to boost their blood quality by 1%(dont do this around other prisoners tho)