Flagellants are the obvious source of robed guys. Other than that you might have to go with a combo of Empire and High Elf parts, for the robe bottoms and shirt sleeves (assuming you want to avoid the split sleeves the Empire generally has.
If the old Middenheim sprue is still available somewhere then that's a good source of hammers.
Empire archers have a sort of tunic thing.
If you're looking for pilgrim style Witch Hunter heads that could be tricky. I can't think of any current GW versions. There was a metal special character years ago, but tracking down 6+ of them to harvest heads from seems like a lot of work.
Confrontation Darkness Hunters aren't really going to mesh scale-wise with any GW parts.
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