Unfortunately we don’t carry parts for these welders available any longer. We recommend checking with a local electrical supply shop.
Those capacitors are 370V. A couple of places where you might find them are: Galco Industrial Electronics (800) 575-5562 Packard, Inc. (800) 334-1769
You may find this document useful: https://www.askforney.com/files/8714/1107/6686/Spitfire-Sl225-Welder-Manual-And-Schematics.pdf
For a lead 25′ or less, you can use a #4 wire. For longer leads, a #2 wire would be better.
If it’s the black nut on the outside of the output jack, use ITEM# 57501.