First of all, which of the souls have you gathered? What do you know? It's all good questions. And I have found all souls in PC but none in Mobile (cuz I'm not in Hardmode in Mobile).
Kill Wyverns in the sky in Hardmode for Souls of Flight.(these are required in order to craft wings) They spawn among Harpies, but are much rarer.
In Underground Corruption/Crimson kill monsters inside it in Hardmode to aquire Souls of Night!
Next is Light, which are dropped by monsters in the Underground Hallow.(full name: Soul of Light)
Then for the other souls: Soul of Might is dropped by Destroyer(100%) and rarely the mobile Lepus.
Soul of Fright is gathered from Skeletron Prime, and Souls of Sight are dropped by The Twins.
There is also a Console/mobile Soul Dropped by the Console/Mobile Ocram called souls of Blight.(Ocram is tricky, so be prepared)