My teams hand out t-shirts Google has printed at the various conferences/events we attend.
But I suspect *most* of the Flutter t-shirts you've seen have actually just been printed by others from our logo assets which are all public:
My understanding is it's generally much cheaper/easier to just print new t-shirts locally, instead of bothering to ship them to new places. If you don't want to print a whole batch yourself, I'd encourage you to talk to a local meetup organizer, or GDE as they may have interest in having a bunch to hand out, etc.
Hope that helps?