I've now found a site that sells them in my country and lo! Oh dear, so many other things I've never heard of to try too.
'Thanks?' :)
Which I would say is besides the point, the article was about recreating an ancient mead based on a recipe, which called for those ingredients.
Sometimes it's impossible to source the historical ingredients because they themselves have been lost (to memory or possibly entirely e.g. silphium), but that's not the case here. Both grains of paradise and long pepper are in active use, even if not in european cuisines.