Here are some great resources for out-of-print and hard to find Christmas music.
Fa La La La La post some really great music and the forums are a great place to meet people and share music. Download the 2004 and 2005 ADVENTures In Carols compilations. They are WONDERFUL!
Ernie (Not Bert) has some really great music too. Make sure you look through his archives of last Christmas too. You can find gems like these over there. I've been trying to hunt down this Santa Claus Conquers The Martians album for a while now...
Visit the Captian over at the Christmas Yule Blog too. Great stuff there. Even an Arnold Schwarzenegger Christmas greeting.
I visit Senses Working Overtime for Christmas clip art that is second to none. They also share some great music that is categorized into fun themes. Check them out too.