Online Resources. A wonderful place to start your research is the book Dragons and Dragon Lore by Ernest Ingersoll; you can use that book to get all kinds of ideas for further research, and you can also find many other dragon stories at the Sacred Texts Archive site. For dragons in fairy tales, check out the dragons in Andrew Lang's fairy books.
Wikipedia is also a wonderful resource for some of the most famous dragon legends, such as Saint George and the Dragon and Beowulf and the Dragon, and the Wikipedia article on dragons can be useful as well.
For Biblical dragons, be sure to check out Daniel and the dragon in the Hebrew Bible readings for Week 3 in our class.
One of my own personal favorite dragons is the Polish dragon of Wawel Hill: