You don't build a flashcard deck by hand and you don't buy one separately. As you work through a course module, the concepts in it are turned into flashcards for that certification's deck — by the time you finish a domain, it's already drillable.
A flashcard that just confirms you were right or wrong doesn't teach you anything the second time you see it. Every card shows the answer and the reasoning behind it — the why, not just the what — so a wrong guess still moves you forward.
Flashcards are the one mode on the platform with no timer and nothing that looks like a grade. The point is repetition until recall is automatic, not performance under pressure — that's what exam mode is for.
Miss a card and it returns to you more often until you stop missing it. Cards you consistently get right drop out of rotation, so you're always spending time on what you're actually weak in.
If a domain in exam mode or training mode is dragging your score down, jump straight to that domain's flashcards rather than drilling the whole deck.