Suggested Farm #1: Exit from Ran Musu Gardens into Minister Cho's Estate. Head east to the area where you finish the mission. From there, follow the mission path backwards to the beginning, then backtrack a bit until you can head east. The entire eastern area here is full of Sickened and a few Afflicted.
Note: The Sickened animals don't drop Trinket Boxes, so there's no point in clearing the western shore area. The Afflicted also don't drop the boxes, although they are often grouped with Sickened humans (most commonly an Afflicted Necromancer with two Sickened Guard rangers).