Two places you can could go and both offer different sorting capabilities:
Direct to iTunes - has the different grade levels in separate tabs: need to brush up on your nursey rhymes? look under K, some of Aesop's Fables are under Grade 4, and under the Grade 12 you have Aristoltle's Physics.
Lit2Go - If you go to this site you can search by author and title, plus you can download a pdf of the reading passage. I also noticed that there was some support material giving suggetions for how to use the document in a class.
I read through the FAQs and saw these two interesting points:
Reading levels are determined by Microsoft Word and then verified by classroom teachers.
According to Microsoft Word Help:
The formula for Flesch-Kincaid Grade Level score is:Copyright - you may use up to 25 mp3 and/or text files for education purposes with appropriate credit.
(.39 x ASL) + (11.8 x ASW) - 15.59
ASL = average sentence length (the number of words divided by the number of sentences)
ASW = average number of syllables per word (the number of syllables divided by the number of words)