The "Supplement" is of course nothing more than the National Archives microfilm record cards, typed up and printed. In fact, if you are researching Union / Federal units that stayed in service past the end date of the "Civil War" the "Supplement" editors didn't bother to type & print those last months (or up to over a year), nor do they inform that those records exist. Nevertheless, it's a convenient resource if you have access. Perhaps the most helpful feature is the index volumes. But the "Supplement" is nothing new.

Anyway, those "original" microfilms are viewable and downloadable at "Openlibrary [dot] org, and/or "Internet Archive", search for "Compiled Records". It looks like most (if not all) reels of National ARchives films are available. For some reason the reels seem to be in reverse alphabetical order. You ought to download the National Archives guides for the records. After you download the reels you need, you will have almost everything you can get from the OR "Supplement" - - - for "free".