But while you can't get the Onesie in real life, you can now get it in your virtual life, as Larry Hryb (Major Nelson) worked with the Xbox Avatar Store folks to bring the Onesie to your Xbox Live Avatar.
In his blog post, Hryb seems to imply that there is no further availability of the real-world Onesie coming, saying the following:
When the Xbox Onesie was revealed a few months ago, I received a ton of inquiries about how fans could get their hands on one of these rather rare items. Unfortunately, distribution was extremely limited and I was only able to give a handful of them away.
We've reached out to Microsoft for clarification on this statement, and we'll keep you updated when we hear something. The Onesie was originally designed to promote the Australian launch of the Xbox One S, but wider availability was promised at the time.
You can add the Xbox Onesie to your Avatar from the following links: