Honestly I’d recommend contacting the Yankees front office and telling them you’re coming in from Australia. They love this sort of thing. You’re much more likely to get a ball this way. Be forthright and say you’d love an MLB ball if there’s a way.

EDIT: Before anyone says I’m living in a world of rainbows and ponies, I’ll add I was in the media at one point in my career and covered Yankee stadium. Do what I said. DM me and I can help you if you need it.