You may need to look for 8.27" x 11.69" - https://www.staples.com/hammermill-copy-plus-8-27-x-11-69-co...

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 don’t know what specialty stores you’re talking about but A4 is readily available at most stationary stores, or anything related to letter writing, pens, or paper. I got the A5 notebook I’m currently using at Barnes and Noble, they also have A4.

Heck, I’m pretty sure you could get a sheaf of it at any number of office supply stores right now if you wanted.

For printers?

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.

Kodak used to have an industrial printer partnership with Heidelberg. They would test their printers with pallet loads of A4. Most I've ever seen in the US.

Yo do know all the jokes about how the US would anything as a measuring system except the metric system? Same with paper.