I can only see them drop in the gardens. So to get some red it was pretty easy, did a few level 8 and every 2-3 level completion I'm rewarded with 6x 3*Evo runes. Which is fair for the SoT and time spent.

For the other colors, principaly blue which I absolutely need now, they don't seem to drop, tried levels 8 to 10 and zero 3*Evo runes, nada...

But if it's just RND, like the death talisman barrier set that I could complete for the 1st time today. I tried to collect that set since I started with the game, got shield only...