Kimberella quadrata
Ediacaran
Late Precambrian (Vendian)
Penega Formation
East Angelsk, white sea, Russia

Large 1 inch CAST of this example of the earliest known mollusc.

Kimberella is a genus of early, bilaterally-symmetrical animal originally described from the Ediacara Hills, southern Australia as a jellyfish, then later reinterpreted as a box jelly.  Since then, numerous well-preserved specimens from the White Sea, northern Russia have led researchers to conclude that it is more closely-related to molluscs (something like a primitive slug).

Specimens are almost never on the market, and when they are, they are expensive.  You might as well get a nice cast like this one instead.

Casts are affordable visual aids to fossil collectors, museum curators, and science teachers.  They are life-size copies of actual fossils which fit well with fossil displays of all kinds.