If you ever get to Kansas City, you can't call yourself a true baseball fan without visiting the Negro Leagues Hall of Fame and Museum. Like all museums, many of the displays are meant to be educational, but rest assured the Hall also has plenty of memorabilia that would catch the eye of any collector.
Start with the oldest-known piece of game apparel from the first Negro Leagues World Series won by the Kansas City Monarchs in 1924. It's a sweater worn by a team trainer or doctor representing the Eastern Colored League.
How about a photo from 1952 featuring teen Hank Aaron, preparing to start his pro baseball career at a Mobile, Ala. train station.