you can get a shard for any act anywhere. Thought a5 shards seems to be disproportionately rarer than the other ones xD might just be my bad luck, but i've found dozens of shards for a1-4, but 0 a5 shards in weeks. lol
A1, A3, A2, A5, A4 has been my distribution from most to least. I just want A4 shards I love farming that act