Store, and from campaigns. I believe that Nightmare does not drop them, the rest do. 

The affinity keep is the other place that drops larger amounts, but campaign is the 

Brutal campaign is probably the most time- and energy-efficient way to get green shards, with the added bonus that it is arguably the most efficient way to get silver and experience as well.

Efficient in terms of real-world time, perhaps. Nightmare 12-3 / 12-6 are more efficient energy per xp and silver.

 Better to do brutal instead of   lower 

It's close enough, especially considering the requirements involved, but primarily nightmare will remove shards from the drop pool which makes it a poor choice for farming shards.

Yeah, I was only remarking on the second half of your statement :)

As everyone mentioned, brutal campaign is an incrediably fast way to get green shards, but you have to be prepared to spend the time in doing it.  Make sure you have 100% exp gain on and use this time to build some food, so that you can knock 2 birds off with 1 stone

Also, why are you so low with green shards?  Are you pulling them a lot during the day?  Ideally you should only be using shards during events

Guess so. I pull 10 a day looking for good rares, alias, got non that can currently help me

The odds of getting rares from mystery shards are very low - and considering how few rares are truly good, the odds of getting a good one are even lower. In the long run, over a period of several months, you'll probably end up getting a few good rares from mystery shards, but trying to force it on short notice is just a waste of shards. You should only be pulling mystery shards when you need additional food champions, or during champion chase / summon rush events to get to whatever score you need for a specific reward.