Its possible to cheaply make a low-voltage, high-current transformer that can deliver hundreds of amps at a few volts AC for spot welding. You'll need to use heavy gauge welding cables or multiple stranded wires in parallel to make the custom secondary winding. You'll also need to fabricate pointed welding electrodes out of pure copper rod, and you'll need to design a way to tightly clamp the steel sheets between the electrodes.
Try to start with the largest microwave transformer you can find - these also appear on eBay from time to time. The larger the core, the more room for your heavy-duty secondary winding and the higher the current that can be delivered.
To disassemble the core, the weld bead must be sawed or ground down. This permits the E and I sections of the core to be separated so that you can remove the HV winding. You'll need to use at least 2-3 turns of heavy gauge flexible wire as the secondary winding and to connect to your copper welding electrodes - #2/0 - #4/0 is ideal. Once you have completed adding the new winding, the E and I sections of the core can be clamped together and/or welded.
The following videos show how large microwave oven transformers were "repurposed" in this fashion. Try not to kill yourself or burn down your house...