How to get back a card in Google now which was removely accidentaly?
on the bottom of google now there are 3 icons. Click the magic wand (middle one) and select others. There find the topic and tell Gnow that you are still interested.

New cards will be added automatically once you search for them with google now. Once a new cad appears tap the three dots in the upper right of the card and than yes when prompted if interested.