It's only a couple of minutes that Killua is gone. He's fast, and he just grabs whatever card deck he sees on the first shelf of a convenience store. A long with a few back up things, but they were all within reach.

So with that, he comes back, passes through security and is escorted again to Hisoka's cell. When he approaches again, it's unusually casual how he sits and pulls out the cards. These ones happen to be very stylized, but Killua doesn't seem to care, shuffling them in focus.

"Hey, what kind of card games do you know?" He's got to know some. There's no way he wouldn't.