For "authentic" demitasses, you've got to go Italian! The four manufacturers I'm most aware of are
IPA,
ACF,
Ancap and
Nuova Point. The first three are significantly older (founded in the 1950's or so) than the last. Nuova Point sells direct, in addition to being carried by
EPNW. ACF makes the La Marzocco cups Espresso Parts carries, while
ESI carries the plain white version in sets of six. AFC's US distributor,
Great Infusions, also sells direct.
Italianissimo is the US distributor for Ancap, who made the Elektra and Intelligentsia cups people have liked in the past. I've yet to find a source for plain IPA cups, but they make the famous illy-logo'ed cups, widely available (but
Chris Coffee sells them by the dozen) .