The article Kettlebell Grip Strength by Adam Signoretta includes a few very challenging exercises for your grip using a kettlebell.
My favorite of the ones he discusses is the towel deadlift. You can easily do that one with a dumbell or weight plates as well if you lack a kettlebell. Just insert the towel through the plate holes, or double-wrap it around the dumbbell's handle. Either will work just fine, and it's a good way to work up the grip strength required for towel pullups.
1 hour ago