Try a fabric store, and see if the prices are any better. The brand Joann's carries is called Dritz. You might be able to find it on line, I don't know.
I gave up on grommets with my first mischief. I was very proud of my first hammocks, but they were in the cage no more than ten minutes before they ended up on the floor, with all the grommets neatly removed and still hanging from the hooks! My current rats might not do that, but I don't risk it--I just cut slits in the corners for the hangers to fit through.
I watched with great delight as my beautiful sweet Elizabeth went to work one afternoon on a hammock that was already hanging by diagonal corners (they seemed to like it that way, I never understood why). She seemed so surprised when she removed the third grommet and was unceremoniously dumped on the floor of the cage. It was hilarious!