The closest replacement I could find was this Roterfaden model, with a dot grid on the verso and staff paper on the recto. The red stamp on the front was nicely old-school, and the pages were the right size. But it did not have a cover. This was a problem: any notebook of mine is going to be carried around, smushed, beat up, and eventually used as a coaster. Roterfaden sells very nice customizable leather covers, but they are well beyond my budget.
So! I made my own. Here's the process, for anyone who might want to try it. This makes a flexible, tough cover, similar to oilskin. You could incorporate board for a true hardcover, but I think you'd have to do that with library paste rather than sewing.
From some past sewing projects, I already had decent-sized scraps of cork fabric, lining, ribbon for the bookmark, and quarter-inch elastic for the strap. (To make masks this spring, I had to buy 50 damn yards of elastic; all smaller quantities were sold out. I have half a football field of black elastic. Hit me up if you need some.)
Roterfaden's custom bindings include a metal clip that attaches the notebook to the cover and lets you replace the inner pages as needed. After a bit of time perusing r/notebooks, I determined that the best option was probably a wallet clip. So I ordered that from a craft supply shop, and it was the only thing I had to buy new.
I cut front and back pockets from the lining fabric. Each was 12 cm x 16 cm. Mistake #1: they should have been slightly narrower. (This fabric and the cork cover are both from the delightful Oak Fabrics, btw.)
I folded the rectangle over and stitched it to the inside of the cork to set the spine. The clip adds a lot of height to the fabric, so to stitch alongside it, you have to use a zipper foot. Mistake #2.5: I very clearly didn't get the fabric straight here. (At this point I had already had to push away Arthur multiple times, and I just wanted to finish before he succeeded in his baffling but constant goal of eating thread.)