I've seen them on DecalGirl;
https://www.decalgirl.com/skins/Nintendo-3DS-LL-Skins
And you can make customs, just click "Create Your Own" up at the top and pick "3DS LL" as the console (you'll know you've got the right one from the c-stick).
They're listed as 3DS LL skins, so it's a little confusing but from the pictures I believe it's for the New 3DS XL. As you said, I would guess that they will become available on most sites that currently do decals once they are released in the US, so you will probably see more then.
But anyways, hope this helps! c: