What exercises improve tackling for rugby?

Tackle more.  Don't you guys have something which you tackle?  Do that some more.  Then general strength training with barbells.  And sprints.

Thanks, I'll consider buying a tackling bag 

Keep in mind tackling in Rugby is very different than American Gridiron football. You need good core and leg strength, with some upper body strength. In my experience, people who have stronger upper body and weaker lower body tend to high tackle more(Trying to strong arm someone or close line style tackle them). Tackling has a lot to do with technique as well. A lot of backs can get your tripped up if your footing isn't good. Your team should be doing tackle work. A favorite drill of my old team was to make a square "tackle box" out of cones.  The drill trains both the carrier and the tackler. The carrier learns how to react against a tackle - a forward drive or a dancing back and how to place the ball once your put down. As the tackler, it teaches you how to react to juke and spin moves (A fleet footer wing or OC can be an ankle breaker) and how to tackle a forward driving into you. If you team isn't running at least some kind of tackle drills, you should suggest some. In Rugby it isn't always about tackling the hardest. A good tackle sets up a good ruck

Doing squats and deadlifts would make sense