What are those curved corner blocks for shoe molding called?

Those were standard issue on a lot of houses from the 30s through the 50s, exactly like you describe. A little quarter circle block that sits in the radius corner and the shoe runs die into it on each side. I've never found them sold as a stock item anywhere either, and I've asked at every lumberyard and millwork shop I've dealt with over the years. I think they were shop-made back then, cut on a bandsaw by whoever trimmed out the house, which is why nobody carries replacements now. I ran into the same problem in my own place and ended up just making them. It's less scary than it sounds. Take a scrap of the same thickness as your shoe, trace the curve off the wall corner onto it, and cut just outside the line with a jigsaw or bandsaw. Then clean it up with a sanding block wrapped around something round til it hugs the wall. The face only needs to match the shoe profile roughly since a bead of paint or stain-grade filler hides small gaps. Glue and pin it in place, then butt your straight runs into it like the old ones were done. If you want to cheat a little, some guys kerf the back of a short piece of quarter round with a bunch of closely spaced saw cuts and glue it around the curve that way. That works better for bigger radii and can look lumpy on a tight one, so test on scrap first. Two other places to check before you cut anything. Architectural salvage yards sometimes have buckets of these pulled from demolished houses, and if your trim is paint grade you could even laminate up a block and shape it with a rasp, no fancy tools needed. Stain grade is harder to fake, so matching your existing oak means making them from the same stock.

Another option if you don't wanna make blocks: the flexible composite quarter round stuff bends around curves like that without kerfing. I used some in a hallway with a wonky corner and it conformed way easier than I expected. Only catch is it's paint grade, so if you've got stain-grade oak you're outta luck there. I'd also say trace the curve on cardboard first before you touch any wood. Makes it way easier to sneak up on the shape.

I gotta push back on the kerfing tip. Every time I've tried sawing the back of quarter round to bend it, the face splits along the bead before I even get it halfway around the corner. The cove is thin enough that those cuts turn it into a bunch of flat facets too, and you can see every one of them once it's painted. On a tight little radius like a drywall cove I'd skip it entirely. When I had this problem in my last house I just made a slightly wider block than the original and let the shoe die into it, which covered the curve without fighting the bend at all.

I ran into the same thing in my dining room last year and made my blocks from offcuts of the original shoe I pulled out, wich kept the stain matching dead on. The part that saved me was a drum sander chucked in a drill. Held the block against the wall corner, ran the sander along the edge, and it shaped the curve to match in a couple minutes flat. A rasp alone kept leaving me flats that showed under the paint, the drum just follows the arc. Butted the straight runs into it and you honestly can't tell now which piece was new.