I've heard Sosa doesn't sign crap through the mail, not that you probably care anyway. I'm planning to send cards or maybe a ball or two to Kerry Wood, Mark Prior, and Hee Seop Choi. I would guess that you'd have better luck with some of the minor stars like these guys before they get huge rather than Sosa, Bonds, Arod, etc. At least that is what I'm going for!
Good luck! Keep me updated on how you do.