[Zarr]

All the three original parts are in the keep.
SPOILER
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Later on though in TOB you can find two extra parts to make it a +5 weapon.
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END SPOILER

Here's the location of the three pieces in the keep
1)near the forge in a container
2)on the body or given to you by Glacius on the second floor
3)in the golem room on the second floor.
Do note however that once you've killed Torgal and cleared the keep any pieces not picked up will disappear so make sure you asemble the flail before you take on Torgal.