As mentioned in another thread, try the Dynaflex:
http://www.dynaflex-intl.com/
It is great for building forearm strength and overall coordination in the forearm to wrist area.
The best exercisers I have is this:
Cut the surface of a vinyl notebook to 9 inches in length, 6 inches wide. Cut two pieces, one for each hand.
Then buy a sofa cushion that is very soft, this is important: harder than a pillow but softer than regular sofa cushion. This cushion should not be more than 14 inches square.
Place the cushion in your lap with the two tops and begin to do heel-toe motion, over and over. This will exercise your wrists and forearm connected to the wrist. This motion is what produces a deep open tone that digs deep into the drum head. Use this for at least an hour a day, while you watch TV and you will see how deep your open tones will sound.
For slaps I recommend the soft balls that they sell as stress relievers. Use them throughout the day when you sit listening or watching something. They develop the grip strength on your last three fingers which are the most important when the slaps crack the drum.