The cards you find in your library must be creature cards. For example, you can’t put a land card onto the battlefield even if it has the same name as a land you control that’s also a creature (perhaps because of an ability of that land).

If you control no creatures when Doubling Chant resolves, you may still search your library and you must shuffle your library.

If you control a creature whose name has been changed by an effect (for example, a Clone that’s copying another creature), you’ll search for a card with the new name, not one with the original name.