The CSC (or CVV2 number) on your credit/debit card is a security measure banks require for all transactions where cardholder is not present. Since the security code is listed on your card and not stored anywhere else, the only way to know the correct number is to physically have possession of the card itself.
Most credit/debit cards display the three digit code on the back of the card, after the card number in the signature strip. On American Express cards the security code is a four digit code and it appears on the front of the card in the top right corner.
If you cannot find your CSC please contact your card provider.