Board Thread:General Discussion/@comment-31283382-20131101162239/@comment-4957813-20131103192330

Isshin's and Urahra's conversation after Grand Fisher was defeated implied that Arrancar can be naturally occuring, though incomplete. It is only through Aizen's intervention that more complete Arrancars could be created.

In addition, Aizen found Starrk and Lilynette after they had already become Arrancar.