I highly recommend listening to anime and then cross-checking with the transcripts to see if you heard it right. You can also use rikai-chan (Firefox) or rikai-kun (Chrome) to check the readings of the kanji easily.
Found this for the Death Note manga:
Here's another site with both Japanese and English transcripts for animes (like Code Geass!):
For Geass: http://animetranscripts.wikispaces.com/Code+Geass%28Japanese%29%3E01.+The+Day+a+New+Demon+Was+Born