Over the years, Scott has sold albums in a lot of different widths, at least five and I think maybe more. They don't seem to have produced just a group of standard sizes as they do today. Album sizes seemed to depend on how many pages an album needed. I have old Scott albums from the 1950s and 60s that are very narrow, about 1.5" wide, and I have Scott albums that are 2", 2.5", 3", and a few much larger than that. These were all 2-post albums. They also produced slipcases for these sizes.
Today, Scott sells three binders, a standard-sized 2-post binder in one size (about 3"?) and two 3-ring binders, one narrow around 1.5," the other about 2.5". These also have slipcases available for them.
Under their G&K brand, Subway Stamp Shop used to make a few different size binders and slipcases for Scott pages. They were very well made and looked just like the Scott binders. Today, they seem to only make the one 3-post binder (and maybe a slipcase to fit it), but neither of the 3-ring binders.
Finding binders or slipcases that match album sizes no longer made is pretty difficult, but not impossible.
ebay is your best bet, I think. It takes a bit of hunting, and many are in bad shape when you can find them. Beyond this, I don't know of anyone making slipcases today that fit Scott binders, but I wish Subway or someone would.