squeeze blood out of a turnip 

You can't get something from a person, especailly money, that they don't have i.e. A turnip cannot be coaxed, squeezed, or cajoled into producing blood. All efforts at obtaining blood from this vegetable will be futile

He's no snitch; you'll squeeze blood out of a turnip before you get any real answers from him.

squeeze blood out of a turnip by BoogiePants October 20, 2008