Barry,
There are some important qualifiers in your question.
Not everyone would agree around here about "good music!"
But let's say you want to get an instrumental version of "Take Me Out to the Ballgame." It's as simple as using a search engine to plug in the name of the song + instrumental.
There are dozens of choices, from free MIDI files to orchestrated versions being sold for a couple dollars for private use. I'm sure iTunes has something and you can hear a sample first. You might try signing up for a trial of eMusic and use your 50 free downloads to search out a suitable instrumental piece.
Organ music and brass bands are typically what have been the backdrop of baseball.
Hope that gives you some ideas -- but what's "good?" Well, you'll have to decide. ;-)
Edit: One tip: Set your camera's audio to 48kHz/16-bit and make sure that if you "rip" this music you match those settings.