All three physical versions of Mega Man X Legacy Collection 1+2 are now available to pre-order from GameStop: Nintendo Switch, PlayStation 4 and Xbox One. The PlayStation 4 and Xbox One versions are 2-disc sets; Legacy Collection 1 on one disc, Legacy Collection 2 on the other. For Nintendo Switch, the bundle will include a cartridge with Legacy Collection 1 and a digital download voucher for Legacy Collection 2.
If you're looking to import the Japanese versions, your best bet at the time of this writing is Play Asia, Amiami or Nin-Nin Game. Amazon Japan are not shipping the games outside of Japan at this time. That may change closer to release but there's no guarantee. UPDATE: International shipping now available from Amazon!
Lastly, I'd like to point out that the standalone physical release of Rockman X Anniversary Collection 2 is indeed shipping on its very own Nintendo Switch cartridge. It's the only way to own the second collection on a physical Switch cartridge because on the 1+2 bundle – both in Japan and the US – it's in the form of a digital download voucher. You might want to look into importing it!