I was recently made aware of a new source of John MacArthur mp3's from Travis Carden. He's collected over a year's worth of MacArthur's sermons so far. This is taken from his website:
Grace to You is the wonderful expositional preaching ministry of John MacArthur at Grace Community Church in Sun Valley, California. In addition to being broadcast internationally on radio, it is daily available via streaming media, download and podcast online at OnePlace.com. Though OnePlace only advertises the 52 most recent broadcasts, they've kept the Mp3's online from all the way back to September 22, 2005! Get them here! ... And if you appreciate this resource, please consider saying thanks.