Best bet probably a salvage vehicle - but it is mounted near the front of the vehicle above the wheel well, so I would guess it is commonly damaged in salvage cars.
What is wrong with the current DC-DC converter? I've never repaired a Model S DC-DC myself, but it's fundamentally a standard device, no different than a conventional off-line converter, no bridge rectifier as it operates off DC not AC. I've repaired those before, so if you need any guidance...