To guarentee that they are authenitc, I would recommend simply getting them yourself from a game. However, if this is not an option or an unrealistic task I would visit MLB Autographs.com, they are the best on the net on Autographs. Simply scroll down to see the link.
Chat with our AI personalities
Get to the ballpark early and stand by the player entrance. Some ball players do stop and sign. Before the game make your way to the field. During batting practice many ballplayers will sign along the line. Both ways, you will have plenty of competition. Be sure to come prepared with a pen, and an item to be signed. TTM - Through the mail. Some ballplayers will respond to autograph requests through the Mail. Send a self addressed envelope along with a photo or index card to be signed, and you might get lucky. send it to the team address of the ballplayer you want the autograph for.