I’ve owned and rather rapidly destroyed a few of the “Gripp” brand hand strengthener balls. They’re great for about a month, but inevitably whatever is inside of them comes oozing out. Instead of continuing the vicious cycle, I spent a couple bucks on several containers of Silly Putty, which is essentially the same thing as the Therapy Putty used in hand/forearm rehab (but not as expensive).
Form it into a ball, then manipulate it to strengthen flexion and extension of specific digits or the whole hand (unlike mechanical spring-type exercisers, which only work on flexion and neglect the thumb). Below are a few sequences that illustrate exercises for different hand movements with martial applications. Pictures 11 and 12 in particular illustrate the putty’s utility for conditioning the hand compression required to support finger strikes. If you’re feeling really adventurous, stick the wad in the freezer for a while before working with it.