I don't see why it wouldn't exist in American servers... Seeing as there are a plethora of drops from the Plague Oni, you probably either A: have a slim chance to obtain it in the first place (especially since it's an insignia hat, which could make it a rarer drop), or B: you are unlucky. I would say to just keep trying because the wiki lists that hat under his drops, and I don't know why the drops would be exclusive for some countries.