Try hand grip exercisers (http://amzn.to/2vZZ0jr ), especially those with adjustable tension for each finger.
Use rubber band(s) (e.g. postal rubber bands about 3 1/2" X 1/4") or elastic tubing (http://amzn.to/2wz3GQJ ) to exercise each finger (or multiple fingers) and thumbs. Hold the band(s) with the opposite hand as you pull the finger(s) you are exercising against the resistance of the band(s). Work every direction the digit(s) will move. Pay special attention to pulling back with your fingers, hands and wrists.