Cavalier's Sword of the Defender is a random world drop, meaning it can drop just about anywhere. It's a level 47 item, so your best bet would be to farm level-appropriate zones.
Data readout for this item includes:
Dropped by: Gutterborn Operator
Zone: Shimmersand
Drop rate: <1%
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Dropped by: Emerald Scale Shifter
Zone: Stillmoor
Drop rate: <1%