Dusk Stone?

[blasteg]

I remembered secret pals having skill "look for stones" might give you one when performing that skill. It can be done once per pal per day.
I have 3 pals in my base, so I get 3 stones a day.

Though based on Murphy's law, you probably get tons of other evolution stone and everstone before the one you want.

Edit: The skill is only available after bronze rank (30 flags), and more of them have it after silver rank (100 flags)