A cheap stick welder gives you a stepdown transformer with a secondary good for 100-200A. That has its own current control (using a magnetic shunt operated by the handwheel) but it's not that controllable.
Another way to get heavy current at very low voltage is to make your own stepdown. Wrapping a few turns of heavy cable through the open centre will create a very low voltage, high current secondary. E.g. a 500VA tranny works at around 2 turns per volt, so two turns of 25mm² will give you a source of 100A or so at 4V. It won't be a perfect transformer because such a sparse winding won't couple well, but it will work. You can make a better job by unwinding the original secondary and filling the space with multiple paralleled cables. 4V is sufficient to pump that 100A through any object under test up to 0.04Ω.
The adjustment can then be achieved, as with the welder, using a separate variable voltage transformer a.k.a. Variac.
