Paul, there are species that rarely/never make it on the seed lists of even the international Clematis societies, so these are the ones that interest me---a (very) partial beginning list would be:
C. aethusifolia
C. fusijamana
C. japonica
C. trichotoma
C. fauriei
C. connata
C. ranunculoides
C. crassisepala
C. dianae
C. nannophylla
C. nobilis
C. robertsiana
C. turkestanica
C. ochotensis (and forms)
and from the American viorna group
C. albicoma
C. addisonii
C. coactilis
C. gattingeri
C. moorefieldii
C. reticulata
C. socialis
C. viticaulis
C. versicolor