To answer your first question, I don't remember ever seeing "filler strips" (I call them "page spacers") sold for the blue Scott International albums, and Amos Media doesn't sell them today. They do sell spacers for the green rectangular post Specialty albums, so I've always wondered why in the world they don't make them for the other album? I've even written to them asking this (and other) questions. Good luck getting an answer.
Yes, you can make spacers, (sorry "filler strips"), but be aware that whatever color of cardboard you choose WILL be visible to some extent between the pages. At first I just used whatever cardboard I had lying around and while that works fine, grey cardboard spacers don't really improve the look of an album with white (or off-white) pages. I'd use white cardboard which is what they use for the Scott Specialty album spacers. Spacers for the Scott Specialty albums aren't a bad bargain, by the way, since you get a couple dozen in an envelope for maybe $10, and they're cut well and have the holes properly cut.
You can punch a few round holes to produce a "sort of" rectangular hole, but honestly that doesn't end up looking very good. Actual rectangular holes also won't slide around. But that's for Specialty albums, and we're talking about International albums now which have round posts, and round holes can be made with any hole punch. To punch the holes accurately, use a filler strip or an album page as your template.
As for which cardboard to use, you want something like white shirt cardboard (from the dry cleaners -- do they still do that?) or maybe you can buy some at an office supply store. Don't use thick cardboard. It's much too hard to hole punch, and it will space the pages too far apart and look very strange. It's best to use spacers more often than to use fewer spacers less often.
A paper cutter will make cutting the cardboard strips much easier and you'll get the widths more accurate that way than if you have to resort to using scissors. And get ready for painful hands that way. Hole punching is a good workout for your hands, too, so think of the health benefits.
What else do you have to do during a pandemic, anyway?