No matter the time of year, it's never too early to get a head start on your harmonies for the Christmas season. The only setback may be finding the music to do so, given the internet and music stores aren't as generous as St. Nick.
The best Christmas carols can be found in a variety of places. If you'd like to invest in some sheet music to use while you carol around town, your best bets are Amazon.com or your local music store. Guitar Center is a great provider of all things musical, and they can put in an order for the music you're looking for. Amazon.com tends to have a tons of titles, at low prices. Just remember: online orders take a few days to arrive, so don't order them on December 24!
To print sheet music from home, there are a number of websites to visit. Musicnotes.com is an example, but sites like this tend to have a thin selection and focus more on the classics.
To find recordings of the carols, stop at any CD retailer, or visit iTunes. Borders is great for the carols you haven't heard in years, since most stores aim for kids' Christmas songs. If you choose to look online, stay legal -- stay away from Limewire and other music pirating sites that can Scrooge you into a bad holiday season.
You have the tools to find the music, but some people just don't know what to look for. If you want old-fashioned, perfect-for-hot-cocoa tunes, go with strictly instrumentals by any composer. Handel's Messiah is the Holiest of Holy examples.
If you want instrumentals, but not the naptime feeling of the classics, look for some rock melodies. The Trans-Siberian Orchestra is the most famous provider of said genre, and they even perform live around Christmastime.
Vocals are great for those who sing along, and you have everything from the classics (White Christmas by Bing Crosby; Winter Wonderland by Dean Martin; Have Yourself a Merry Little Christmas by Frank Sinatra) to the newbies (Grandma Got Run Over by a Reindeer by Elmo and Patsy; All I Want for Christmas is You by Mariah Carey; The Chipmunk Song by Alvin and the Chipmunks). If you easily tire of just one voice, look for albums that sport the phrase "Various Artists" under the title, like the album NOW That's What I Call Christmas. Compilations are great for mixed-age holiday gatherings because they give you a taste of different songs and styles, all for the price of one.