At one point on an international project I had to fly a box of UK A4 to the USA in my luggage so the Americans could check their software could cope with the different size. It did, but lugging it around was a pain - paper is heavy!
Heck, I’m pretty sure you could get a sheaf of it at any number of office supply stores right now if you wanted.
Unless you have mobile paper shops. Could be handy, but seems a bit niche.
Or you can buy a ream of legal-size and have a printshop slice it down (which is how I got ahold of B4 or B5 IIRC).
I buy A4 notebooks all the time. I use fountain pens, so many of the notebooks and even loose paper with the proper sizing (coating, that is) usually come in EU sizes. Tomoe River... Clairfontaine... etc.