It's not 'stress balls,' but Chinese hand excerise balls helped me.
I especially found them useful when I broke the
5th metatarsal in my left hand.
They got me up a playing again much, much quicker.
IMHO these really can improve strength and dexerity.
They can be found in Karate Supply stores,
Chinese Medicine markets, and even sometimes at
Sports shops.
Chinese Baoding Balls, were developed during the Ming Dynasty (1368-1644 AD) and are revered for their healing properties with Arthritis, blood circulation, hypertension, and muscle/joint relaxation.
The exercise will keep all of the points in one's hand in constant motion, with the muscles in one's fingers and forearms contracting and relaxing harmoniously.
They are well known for not only their theraputic benefits because of the pressure points in the hands being massaged,
and recommeded for arthritic conditions and carpel tunnel.
Used by marital arts to improve the 'grab arts' and by
sports medicine to rehabilitate and strengthen hands.