-Towel pull ups as mentioned
-Put MUCH bigger grips on dumbells, pull up bars, lat bars, whatever. I mean like 2" diameter grips. Wrap in tape and padding, whatever works.
-Go to an art store and buy a couple pounds of clay. I'm serious. It can be oil or water based. Knead that shit like bread on a scrap piece of board, getting all the air bubbles out of it. As a rest, make something silly out of it, then go back to kneading it. You'll be able to break the spines of lions with your hands inside of a couple weeks.
i fucking suck at BJJ and people comment regularly on how strong my grip is. It comes almost entirely from working clay and wax for a sculpting job i used to have.