As with all things, if there's enough demand, someone is eventually going to make the right size tube. The problem is that there isn't a lot of demand for smaller road wheel sizes, so there aren't that many 650B products for road-width tires in general. And TPU tubes are a newer niche product.
You can contact manufacturers asking if they make an appropriately sized tube. This might let them know that there's potential demand.
I have squeezed a 700c tube into someone's 650c wheel to get them home. I folded the extra section of the tube over. I would not be confident using this as a permanent measure. Also, that was a spare butyl tube, and I'm not confident that a thin TPU tube for a 26" tire would stretch to a 650B tire.
You could expand your search to the mainland Chinese companies that are offering TPU tubes. The issue is that I don't know which ones are credible manufacturers. I have heard good feedback from more than one third party (generally on forums or on Discord communities that I trust) about Ride Now tubes. I have no personal affiliation with the company. That said, they offer a lot of sizes, and it's possible they'll fill a niche for 650B wheels. They do currently list a 27.5" X 1.4", or 35mm tube. There is some leeway with putting wider tubes in. You could try that tube anyway and see if it fits.
Failing this, you might consider going tubeless. Admittedly, this is more complex than using tubes. That said, the new generation of tubeless tires should mount without trouble unless your rim is out of spec. You do likely want a tubeless inflator, or else you could burn a CO2 cartridge to seat the bead (one per tire in my experience), or you could buy or borrow a Presta to Schrader converter and go use a car pump, or ask the bike store to mount the tires. I know that Continental makes the GP 5000 S TR in 650B in 30mm and 32mm sizes. Other manufacturers are likely to have a couple of road sizes for that rim width.
Unfortunately, this demonstrates the perils of using an alternate rim size in some situations. The issue is that 650B works better for some really short riders. Unfortunately, tire molds aren't cheap, so manufacturers may not widely offer appropriate sizes, and if they don't then tube manufacturers aren't going to invest to make the right tubes.