A proper König & Meyer sheet music holder (10068).
It's not foldable, but very stable. If you only ever used foldable sheet music holders, you don't even realize what misery you could simply avoid. Beside not dealing with falling sheets all the time, you can also properly write on it without descending into madness.
If you have to deal with larger pieces, something like the Berolina Manufaktur Magic Music Board will also come in handy.
Alongside with that: A magnet ring you can put your pencil into, so you can attach it to the underside of your sheet music holder. This way, it won't get into the way of flipping pages. Or, if you have to deal with the finicky ones, you can increase the stability of your sheets with it by fixing them with the pencil somewhat.