If you give them your card, they'll naturally give you theirs. Or you can ask "May I have your card?", "Do you have a card?" Note that western cultures are less formal about receiving cards than in Asia. For instance, it's not necessary to receive a card using two hands, and it's not rude to put the card directly into your pocket or wallet, so don't feel offended if you see this happen.

"Could I get a business card from you? "Do you have a business card you could give me?