Welcome to TBN!!!!
There are John Deere forums as well as a Hydraulics forum on TBN. Hydraulics is probably going to be the most helpful for your cylinder questions.
There isn't much to rebuilding a cylinder, but you will need the rebuild kit. Your local Deere dealer should be able to get that for you. If you know what you're looking for, any hydraulics shop (brick store or online) should be able to help you with the parts.