#1: Frozen
I was derping around in the underground snow hallow in early hardmode (something I shouldn't've done but did nonetheless,) when suddenly, bam. Frozen Key Mold from Illuminati Slime.
#2: Jungle
My friend and I beat the three hardmode bosses, and we had started exploring the underground HM jungle. One particular time however, I was down there alone, when suddenly one of the Moss Hornets dropped a Jungle Key Mold.
#3: Hallow
I was fighting the Frost Moon with my friend, when suddenly I found a Hallowed Key Mold lying on the floor with a bunch of other drops. Naturally, I picked it up.
#4: Corruption
I was trying to get the corruption mold for some time, and I created a world and made it 100% corrupt. Then, when I just started building a farm, I killed the first Eater to annoy me, and it dropped the Mold.
#5: Hallow
Random Pixie kill.
#6: Corruption
Random Clinger kill.
#7: Jungle
I think? I don't remember it very well.
#8: Corruption
Random Corruptor kill.
#9: Hallow
I was playing as Celeste, when the first Pixie I killed dropped the mold.