Amazon typically has it; https://www.amazon.com/Hammermill-Paper-letter-Bright-Sheets... by Friday. I occasionally need some for Australian paperwork and forget where I stashed it every time. Some day I'll open a closet and find 20 reams of the stuff with a page or two missing.

I can have 10 reams of Hammermill 20lb in A4 at my door in less than a week for less than $100.

You get it online. I keep it around to print booklets for certain community purposes. I can set latex for A5, design the pages, then print the A5 pdf to A4 paper booklet and Tex, Mac and Brother understand with zero further brain investment from me .. . Letterpaper is a nightmare