Is there a card that lets me draw whenever a card is put in an opponent's graveyard?
Just add Painter's Servant to your combo of Sphinx's Tutelage and Compost.

With Painter's Servant in play, every card your opponent mills will be Black and cause a Compost trigger.  So as soon as the Sphinx's Tutelage trigger first resolves, your opponent will mill (at least) 2, netting you two more draw triggers, causing 2 more sphinx triggers, leading to 4 more draw triggers causing 4 more sphinx triggers leading to 8 more draw triggers, etc etc.  

Compost is a 'may' so you don't need to worry about milling yourself if you want to stop.