2 More great ways of building great finger strength are Chinese exercise balls & bowling balls.
Chinese exercise balls are those silvery balls, usually sold in pairs, that you've probably seen around. While just rotating the light ones in your hand is supposed to be good for your health, rotating heavier ones will increase your finger strength. Find heavy ones looking around the international district, or buy some heavy spherical fishing sinkers at a hardware store.
As for bowling balls, you can find these at garage & rummage sales. Get one you can barely lift and rotate it in a circle. First palm down, then try it holding the ball upright in your hand.
Hope these ideas help,