One of the best ways to get autographed baseball cards is to write to the baseball player that you like. Just send it to the baseball team address located on the teams web page. In your letter ask the player to send you a autographed baseball card. You could send your won card or request one. Enclose a self address stamped envelope to be sure the card is sent to the right address. Some teams offer autographed baseball cards in their home team stores.
There is one way to be sure that you are getting an authentic autograph when buying from an auction site or from a dealer. An authorized dealer will send along a certificate stating that the card has an authentic autograph from the real player. Look for the PSA/DNA certification on the certificate or letter of authenticity.
The certificate shows that an autograph authenticator has examined the card or was present when the card was signed. PSA/DNA uses a unique invisible but permanent DNA marker on every certified collectible (submitted for the premium authentication service), the authenticity can be easily verified. Take a little time to check to be sure it is real, its more valuable that way. If you want to sell it later on you have a better chance to a good sale with certification.
Buying cards from certified dealers is the best option. Check on their site for certification or you can call and ask them about the autographs. Another way to get a autographed card is to have a trusted friend go to a game and get the card signed for you. Then they can send you the card in the mail.