Here are some links to various websites to find Christmas carols!
http://www.carols.org.uk/ – This site is not the best, but it has some recordings, lyrics, facts, and a few games.
http://hymnchords.net/ – Just what it sounds like…chords for hymns.
http://makingmusicfun.net – Go to the “printables” section and you can find Christmas music for various instruments.
http://www.8notes.com – Similar to the prior website. Has lots of free music!
http://holidaysheetmusic.net/ – Carols and hymns.
http://musicnotes.com – This one isn’t free, but it has good music.
I also recommend the Faber Christmas books too. I use the Intermediate Level 4 for myself, and I really enjoy the arrangements in the book; they are difficult enough for me that they sound good and require a little practice, but not so hard that I can’t learn them relatively quickly. All the Faber books are great.