Those hand grips are supposed to work ok. I have a wooden bar that i hold out in front of me at shoulder height, attached to the bar is a weight on a piece of string just above the floor. U have to twist the bar and wind the string up so the weight moves upwards to the bar. Then wind it back down, do not just drop it down, wind it slowly. Keep your arms as straight as possible and you wont do more than 5 reps up and down if your doing it right. Try a 2 kilo weight first and move up if its easy. Really pumps the forearms.
If you have one forearm bigger than the other and you want to even them out try using the 'stranger' method in future.