These are the two best ways I know of to listen to Christmas music online.
First, for a list of Christmas music playing radio stations from around the world, there's Mike's Radio World. From North Dakota to the Netherlands, they've got a pretty good list for your listening pleasure.
Second is for when you want no commercials (ever), and have a specific artist or genre in mind. Pandora. For the uninitiated, Pandora is an online music source which picks songs that are similar to ones that you like. This way, you can create your own "stations" based on a favorite song or artist. For Christmas, I've created stations based on Christmas music of the Mormon Tabernacle Choir, Andy Williams, and Bare Naked Ladies. You can even set it up to select from multiple stations, so you get a little of everything you like.