Do you have an exact model and serial number for the welder? ( photo of the data plate may help )
There are a million different switches, and two million ways of connecting them to the transformer taps. Replacement switches are identified by a manufacturer and a type number. This should be printed on a label on the switch. They are made-to-order. External jumper clips or wires may have to be added by the user. You need to know how many positions the switch has, and how many segments. There are many variations. It is a minefield....
An alternative which is sometimes attempted by brave, or foolish, people is to open the switch and clean, lubricate, repair, or replace the damaged contacts or cams. This usually results in boing, ping, whirr, splat, as springs, contacts, and ball bearings shoot out into all corners of the garage, followed by the sound of much loud swearing.
TecArc in York may be able to help : https://technicalarc.co.uk