Finding the right reading can take time if you don't already have a favourite piece of prose/poem/reading that has significance for the both of you.

Here are some websites I have found that have some lovely ideas...

Sweet and Offbeat wedding ideas 

http://www.senorsenora.com/2010/06/sweet-offbeat-wedding-reading-ideas/

 Words to read when you wed

http://apracticalwedding.com/2009/01/words-to-read-when-you-wed-water-wine/

Light hearted wedding readings

http://offbeatbride.com/2011/06/silly-wedding-readings

Readings from children's books

http://www.confettidaydreams.com/15-romantic-wedding-readings-from-childrens-books/

Wedding Readings from Literature, Movies and Opera

http://weddingreadings.blogspot.com.au/2008/08/wedding-readings-from-literature-movies.html

How to choose readings

http://leladavidson.hubpages.com/hub/How-to-Choose-Readings-for-Wedding-Ceremonies