Static holds, suggested by Chaos, by all means. Preferred in a power rack or whatever. Try the 15-20 second holds with a thumbless grip for awhile, keeping the bar away from the body. Can use DB's also. Pinch grip chins from a rafter or overhead 2X4. When you can do that from a 4X4 you should have great gripping power. Reverse BB curl, thumbless grip. Finger curls, off knees while sitting. Can also do these standing with DB's.
Lot's of stuff anyone can do if not having the extra bucks to buy a quality Gripper. Good examples offered by Magoo, though not sure what a double overhand grip is. Maybe I am doing it by another name, who knows? Good Luck.