Those grippers are okay, you can get some good ones at places like APT here's
http://www.prowriststraps.com/hand_gripper_hand_grip_exercise_gripper
I have unique wrist developers from a guy known here, it mimics the motion of a reverse bend (bending steel, nails, etc) and is pretty good to use.
When doing biceps do some hammer curls as that will help hit the forearms, you can also add in wrist curls and reverse wrist curls. Heavy deads, shrugs and others use a lot of grip strength.
For strength there's many things you can do. If you can afford kettlebells and can fit then into your routine then these will help. Plate pinching is another method. Hanging from things also works your grip, like for instance when doing abs do hanging leg raises. Easy and cheap ways is doing pull-ups with two towels. Throw the towels over the bar so the both ends of the towle are hanging down equally on both sides, grip them and get started doing pull-ups. Ropes are also good for this. You can also use a one hand on the bar and one hand on the towel method and then chance hands every set. Another one to try with the towel is drench it in water and then try wringing it out, twisting and knotting the towel anyway and everywhere to try get out as much water as you can. This works the grip in many ways and doesn't cost anything but the price of the towel.
Farmers walk is a great exercise, you can use a barbell with an handle, dumbells, etc.
More I'll quickly mention is rope climbling, you can climb poles if you haven't any thick rope. There's poles sticking out of the ground almost everywhere you can use. There's so many other stuff you can do, sled dragging, sledge hammer work, etc.
