Actual GM screens in A4 aren't especially common (mine is an American one, so it's designed for their standard 8.5"x11" instead), but there are alternatives.
Sheet music holders, such as this one on Amazon, are an option. I have a much cheaper one (roughly £4 if I recall) but this model seems rather more sturdy. I've heard others recommend menu holders from restaurant suppliers, which seem to be quite similar. Alternatively, a slightly different approach is an easel binder: these are ring binders which form a landscape format easel, but you can also get some that fold out while in portrait, so they display a double page spread comfortably.
@Guvnor I'd be interested in one. Postage wise, they'd probably fit in Large Letter unless the envelope was really padded, I think?
@First Age if you want landscape height you could go with A5 instead. You'd have to cut each page in half and have two (or more) holders, but it's the right size. You could also go with the 2-sheet A5 option as well.
Had a look at the actual GM screen options on Amazon (like this, or this), and I'm not that impressed. Kinda pricey for what you get, they definitely fall into the "only if someone buys it as a present" range (£40-£100+!)
I grabbed a pack of the 2-fold A5 menu holders, stuck in a Monsterhearts playbook I had laying around from a con game:
A bit more classy than just a folded bit of paper. I'm going to try making some A5 versions of con game characters, and putting the illustration in full page on the outside for all the other players to see.