You may know this, but you can buy them for a few arena tickets.
The Lustrous Pip Opal (10%) costs 275 arena tickets from Jordo Mistborn. He's outside the arena, just to the left as you come into Unicorn Way from the Commons.
One tournament will likely give you enough tickets or almost enough just for turning up. I think the minimum we get is 250.
I always buy these instead of farming for them, I find it much quicker and easier.
