Hello Omniwatch,
When you say seam punch exactly what are you talking about? A flanging/stepping tool/ A hole punch? The one nashmetal provided a link to does both and there are hand operated varieties as well or are you talking about the sort used with an anvil or dolly to close a folded seam?
The latter can be made very easily if you have a welder and the slot can be made with a mill, lathe, shaper or a hacksaw and file if needs be.
If you could describe your needs in more detail there is bound to be someone who knows the exact tool for the job.