IIRC Sealand sells post cards, as part of its general merchandise thing (along with nobility titles). Not sure about stamps though. Another quasi-governmental curio is that, again IIRC, Spain has several , maybe even a dozen or something, local currencies. All being traded just fine in their communities. (I think the US also has or had at least a couple, though those were essentially just checks of some co-op banks,…
I've lived in Spain for decades, never heard of such thing. Depending how you count you could argue there are a dozen spoken languages, but currencies? That requires a big citation.