The easiest way to find and browse the ELL Graded Reader collection online is to search them in the Books & eBooks Catalog from the Library's homepage. Scroll to the bottom of the page and find the link in the section titled Using the Library.
Once in the Advanced Search of the catalog, you can click on More Options to find the options to search by Call Number.
In the More Options search, you can change the drop down box to the right of your search boxes to search by Call Number. Call Numbers are the addresses of where the books live in the library. All of the Graded Readers are in the PZ section, so searching for PZ in a call number search will bring up the full list.
It's important that you also check Books in the Item Type list. This will filter out any random items the catalog finds so that you only see actual books. Sometimes the top results will be about the Library of Congress system (which is how our books and other items are organized) but scrolling down should get you to the readers. You can also use the drop down menu on the right to sort your results by Call Number instead. This should prioritize the PZ call number book, which are all graded readers.