Any boat shop should have after markets if its a dash type switch. If its wired to the control handle then find a dealer that sells the make of your outboard.
I'm not sure what kind of switch you are looking for explicitly, but any auto parts store will have a dozen different ignition switches on hand. If you're up to the challenge of fabricating some kind of mount, or modifying the existing switch mount you might be able to save a bit of money by using a generic automotive switch.
The guy that sold me the boat gave me an ignition switch. It just doesn't fit in the hole. I suppose I could fabricate something. It's probably an easy fix. I just need to get out and do it. My problem is I'm always afraid I might make it worse than it already is.
I personally am not allowed to own tools, I like you end up costing myself twice the cost of a repair after I try to fix something.
I recommend Staff Jennings in Eugene, they have always treated me fair and quick turn around times. Good luck and viva Visa
Chuck