The source is Arthur Lovejoy with his The Great Chain of Being: A Study of the History of an Idea (1936), page 50.
The reference is to Timaeus, 30c-d :
But we shall affirm that the Cosmos, more than aught else, resembles most closely that Living Creature of which all other living creatures, severally and generically, are portions. For that Living Creature embraces and contains within itself all the intelligible Living Creatures, just as this Universe contains us and all the other visible living creatures that have been fashioned. For since God desired to make it resemble most closely that intelligible Creature which is fairest of all and in all ways most perfect, He constructed it as a Living Creature, one and visible, containing within itself all the living creatures which are by nature akin to itself.
See Lovejoy, page 52 :
It is this strange and pregnant theorem of the' fullness' of the realization of conceptual possibility in actuality, that, in conjunction with two other ideas usually associated with it and commonly regarded as implied by it, is to be the principal topic of these lectures. It has, so far as I know, never been distinguished by an appropriate name; and for want of this, its identity in varying contexts and in different phrasings seems often to have escaped recognition by historians, I shall call it the principle of plenitude, but shall use the term to cover a wider range of inferences from premises identical with Plato's than he himself draws; i. e., not only the thesis that the universe
is a plenum formarum in which the range of conceivable diversity of kinds of living things is exhaustively exemplified, [...].