How to Write to Professional Baseball Players
Go to the website of the player's team. Links to websites for all the Major League teams, both in the American and National League, can be found at mlb.com under the "Team Sites" link in the upper left side of the link bar.
From the team website, scroll to the very bottom of the page and look for a link that says "Help/Contact Us" and click on this link.
On the next page, click on "other." You may need to click "other" twice on some team's pages.
This will bring up a contact form. Scroll to the bottom of that page, where you will find the address of the team.
Address your letter to the player by name first, then under that, the name and address of the team. The team office will forward your letter to the player.