Where can I find more Element Zero?

Like the other answer, there are lists you can use to find planets for any particular mineral.

If you don't want to use a list, there are a couple of tips:

• Asteroids generally have Eezo.
• Wave your scanner over the planet, if there's any Eezo there, there'll be a lot, and it'll show up immediately.
• You only need ~50k (I think, forgot) to research every single upgrade which needs it, so don't keep mining afterwards (unless you need to respec a lot).
• Game completion bonuses should give you more than the above amount, so you won't need to mine Eezo at all.

It's pretty rare, but there are lists floating out there of planets where you can find it. For example, here.