That should be an SCREW, not an “pin”. Anyone who has an assortment of replacement stainless steel case back screws would have a screw that would serve the purpose. Possibly would be necessary to shorten the screw a bit. These screws regularly go missing during slap-dash sloppy battery replacement jobs. Replace the buckle is an option, but not the best choice. Check a few watchmakers. Battery change counters in big box stores or most retail jewellers won’t be able to help you.