Quest mode deck customization

[Haytrid]

I like quest mode but wonder if I'm missing a feature or option. I want to work towards building my own custom decks but I don't see any way to go after the cards I need for my decks and they never come up in the spell shop or packs.

My decks end up having a random collection of decent cards but not much character and I lose interest in playing them. I see the spell shop has "full catalog" but of course you can't make purchases from there.

Is there a method I am missing to go after specific cards?

Kind regards.