With ecxas disabled, and Breezy Cave gone (which alsl had a very low spawn rate of mirror shards) they are currently unobtainable.
I think making wild Mr. Mimes hold them would be idea, because:
1. They wouldn't affect the excas.
2. It does take some work to collect them.
3. We seriously need a good solution.
If you ask, why Mr. Mime? Well, a mime imitates you, so, just like shellder hold pearls, it seemed a bit cool.