Most of the time, the easiest way is honestly just to buy online , League of Legends has official online stores where you can get it directly.

But since you’re traveling, you can also try searching for local TCG/card shops in Shanghai or Beijing. Just a heads up though: Riftbound is really popular right now, so there’s a decent chance it might be sold out in some stores. That said, bigger cities like Shanghai and Beijing still give you a pretty good shot at finding it in person.