The Charles Sumner correspondence is available on a microfilm set published by Chadwyck-Healy, which includes all the letters from Sumner’s papers at Harvard as well as many letters from other institutions. Digitized versions of the microfilm edition of Sumner's correspondence are available. Please visit the Princeton University Library's digital repository for the Papers of Charles Sumner for access to the digitized versions.
Because the microfilm is organized by date rather than the name of the correspondent (as are the physical papers), the documents on the reels are in a different order than the physical papers. In order to find out which reel you need, consult the reel guide.
Some of the correspondence has been digitized and can be accessed through the finding aid.
If your project requires access to the original letters, you can submit a request and include the reason you need to use the manuscripts rather than the microfilm. The curator responsible for the collection will then reply to let you know whether your request is approved.