Hi, the easiest way is to search trash bin in condominium, flat and apartment. These are the only location where you have a chance to find the card. Take note as you accumulate more cards, the harder it takes to find the cards. So, you probably should this during weekend when you have free time.
Every time you search trash bin, you'll lose a mood. You can recover your mood by drinking coffee at the office (3 daily quota for male & 1 quota for female) or playing games on your desk at the office.
When you search trash bin at apartment's basement, sometimes you can find a bum after 7 pm. Talk to him. He might have a card to sell for $20. If he doesn't have a card, you can repeat the process by exiting the apartment & reenter the apartment to talk to him again.