The hardener is benzoyl peroxide.

The same stuff as Bondo for car repairs.

You might find some extra hardener available at a body shop supply house, or just purchase another can of filler and use the new tube.

There is more than enough in each tube for the contents of the can if used carefully.

Yes, you can buy new hardener. I saw some at Lowe's last week. A large tube was about $5.