What can I sacrifice for Champion of Stray Souls' ability?
You can only sacrifice creatures that are on the battlefield and you control.
From the comprehensive rules:

701.14. Sacrifice
701.14a To sacrifice a permanent, its controller moves it from the battlefield directly to its owner’s graveyard. A player can’t sacrifice something that isn’t a permanent, or something that’s a permanent he or she doesn’t control. Sacrificing a permanent doesn’t destroy it, so regeneration or other effects that replace destruction can’t affect this action.

A permanent is a card on the battlefield, such as creatures, lands, artifacts, etc.. A card that is not on the battlefield can never be a creature (land, etc), only a creature card (land card, etc.).