The Extensive Reading Foundation: filled with all the information you might need to start up an extensive reading program. From lists of graded readers to the Guide to Extensive Reading, the ER Foundation site has most of what you’ll need to start your ER journey.
ER-Central: 1000s of free texts you can use in an ER program, games to help students learn the vocabulary they pick up while reading, and even an on-line graded text editor to help you make your own graded texts.
And if you’re looking for some texts to supplement your ER library, might I suggest:
Simple Wikipedia: there are all kinds of short and long articles on almost any topic your students might be interested in. You can easily select to have articles printed out in PDF/Book format. While the English is ‘simple’ and the articles sometimes shorter than a standard Wikipedia entry, they actual difficulty varies quite a bit from article to article and you might want to run them through ER-Centrals online graded text editor to see what level any specific article is written at.