The easiest way to find and browse the ELL Graded Reader collection online is to search them in the Catalog. You get there by clicking on Find Books and Media on the Library's homepage, under the Find section.
Once in the catalog, change the drop down box to the right of your search box to 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.
The graded readers in the PZ section are sorted by level, with each level of each series in boxes. They can be browsed as well by going to that section on the second floor of the library. All the shelves are in alphabetical order by those one to two letters at the beginning of the call number found on the spine or cover of the books. In the case of the graded readers, a basic version of the call numbers are on the outside of the boxes, with the full call number on the front covers of the books.