There are some alternatives. I would recommend: Ghostly Skull, Blighthawk, Spectral Porcupette, Crow/Raven x2, Vampiric Batling. Bats, Owls, Spectral Kitten could work as well. None of these require real life money, although the Spectral Kitten and the Vampiric Batling might be difficult/impossible to acquire.To counter, I wish to bring up a question I asked a bit ago on the Battle.net forums, asking about a darkness team, since I was having trouble beating them. To my shock 20-some people all replied that I need Pandaren monk or Mini Tyreal/Soul of the Aspects to even consider building a team around darkness (as well as a Crow/Raven but that was why I had asked the question int he first place).
You could use another Darkness provider or a random pet in the third slot. Pandaren monk or Mini Tyreal/Soul of the Aspects are in no way needed for a Darkness team. Crows/Ravens will get you far (they are, in my opinion, the bread and butter of a Darkness team), but everything else is up to your own preferences!