Context:

The shirts (and their expressions) were designed by them in the Jackbox 3 collab stream during the Tee K.O. minigame.
The link of the game results was eventually shared which mean that these shirts actually be bought through jackbox.tv
I'm not sure how long these results are kept though.

Ina'nis Sana Fauna Amelia Gura
Tee K.O. game results
JACKBOX with friends @ 34:40