There are various affixes that can appear on gear such as Confident and Diligent that increase maximum discipline. I've only been able to find Hatred regen in D3db, so it looks like I may have been mistaken about that part in my comment. You can stack a little +discipline gear to give you a bit more of a buffer, but that will only help so much in prolonged fights. These affixes seem to be available from the very start of the game, at least according to D3db. I can't remember exactly when I started noticing them on my own play through, but that's down to luck anyway.
There are also a few passive abilities that can help, depending on what skills you use in your build:
Other things that may help:
- All of your
Defensive skills have runes which reduce their cost.