This quickly became a discussion of how to test paper for acid/alkaline qualities but the original question was about finding a suitable binder to supplement Scott International binders. While pH is certainly something to pay attention to, the binder question is not getting answered. Here are my thoughts.
I do not use 3-ring binders for any parts of my collection, though I know many collectors do, especially those at the more "thrifty end" of stamp collecting. Three-ring binders look cheap to me which is why I don't mount my hard-earned collection on notebook paper or any other 8.5 x 11" paper. I want my stamps displayed as well as possible, so I want albums with some elegance to them. I may not be rich, but my collection can look like I might be. My albums of choice are Scott Specialty and International albums which are the less expensive of all the major albums. I chose these after trying many other albums, Lighthouse, Davo, Schaubek, and others partly because they were less than half the cost of the others in most cases, but also because they look good. I value being thrifty just as I want my collection to look good.
There are many excellent, high-quality albums out there, but cheap albums/binders for stamp collections that are good albums/binders are not easy to find. In search of cheap solutions, a lot of collectors migrate to Amazon
or their local office supply store. This is where, I think, they go wrong. They may not even know there are other options.
Just about the only good-looking non-stamp album (office supply store) 3-ring binders I know of are the "We-R-Memory Keepers" binders sold on Amazon
or the 3-ring binders Amos Advantage sells. They have a "regular" binder that's among the best looking of all, much better than what you'll get at office supply stores. No slipcase is available for either of them, unfortunately. I always use slipcases to protect my collection. I'm thrifty, but I want to protect my stamps.
My point is that not only are cheap methods of storing your collection risky -- hence this discussion about pH testing -- but they also may not look so good. Like I say, your collection doesn't have to look like you're being thrifty if you find good-looking ways to house it.
I have to assume that publishers of most popular stamp albums have made some effort to make them unharmful to the collections we put in them. My main hesitation with these is album binders made of vinyl since I've seen many kinds of plastics degrade badly. I've never seen cloth or whatever paper Scott covers their binders with degrade badly. If they did, very old Scott (or other) albums would be noticeably bad for stamps. They aren't. I've owned some 100 year old Scott albums that have perfectly intact covers. It's the pages I worry a lot more about, and paper does degrade. Of course, by that point (2121!) most of our collection are likely to have been transferred by subsequent owners to other albums more than once. If I knew my collection had to survive in excellent shape for centuries, I'd probably give up collecting because I couldn't possibly afford the materials necessary to house a collection safely for that long. If my collection survives for half a century, I'll be pretty satisfied. Nevertheless, I've seen many 100-year old collections that are perfectly fine, so I'm aware but not particularly worried.
As for which binders to use, when I do use ring binders I use 22-ring binders. These are widely used in Europe and the UK and they look much more classic than schoolboy 3-ring binders. Prinz in the UK sells some as do other sellers like Dauwalders Stamp Shop. If you must use ring binders, they would be my first suggestion since they look so good. And paper is easily available for them, blank or quadrille whichever you prefer.
Lighthouse and other album makers also make ring binders with various numbers of rings (or posts), but you must use their proprietary pages punched for those binders. For Schaubek ring binders, you'll need 6-hole pages and only Schaubek sell them. They are very good pages, by they way, as are Lighthouse blank pages and Davo and so on. I bought stacks of blank pages for some Marini (Italy) binders I bought cheaply. Good paper is out there if you look.
Scott blank pages are punched for both their two-post binders and their 3-ring binders. The best looking of the three-ring binders, in my opinion, are the narrower of the two Scott ring binders. They also sell a large three-ring binder. Though thriftier because it holds more pages, its massive. To me, it looks a binder in an auto parts shop. The smaller of the two 3-ring binders Scott sells matches their other Specialty binders and may be the best choice of all. It is a stamp album binder, after all.
You say there are no "all-blank" Scott-size album pages, but there are. Subway Stamp Shop sells all-blank pages for Scott binders as does the provider of already-printed Steiner pages, though his paper is a different shade of white than Scott's: http://www.albumpages.net/
With the latter, you choose 2-hole or 3-hole punching, but can't have both. With Subway, you get both. If that matters.
Have fun whatever you do!