You might want to expand your search to include forensic USB devices, that's a arena where people absolutely want read only data acquisition, and that might help find what you want.

"Forensic bridges"

There is complexity to read only modes. Depending on the underlying technology the read-only switch could be implemented in software, or the host needs to honor it by protocol, but not physically. That's pretty common for SD cards, if the host computer wants to write to them it can

Some of the better USB sticks, the Read only button, actually prevents the write enable signal from physically reaching the storage, that would be best in class. But you need the schematic to actually verify that. So you don't know if it's actually just implemented in software

For the forensic bridges, they actually speak the USB protocol, because it's just a serial bus, and they simply don't relay any commands they believe are related to writes. That relies on them enumerating every possible serial command, and that both the talker and the listener both have the same understandings for the same commands... It's pretty good, but there is room for error

Most encrypted USB devices, the ones with the keypads on them, have a read only mode. If you trust their software: https://www.kingston.com/en/usb-flash-drives/ironkey-kp200-encrypted-usb-flash-drive

Honestly, your cheapest option is to get cheap USB drives, image them. Put some red gaffers tape on them, whenever you break the tape to plug them into a device they're now tainted, and you as a human must reimage the drives again before you put them into another computer.

https://github.com/o7-machinehum/ovrdrive Here's a fully open source flash drive, if you look at the schematic you can see you just want to be able to disable the right enable pin. This drive is designed with some fancy controller in front of the USB controller so you could actually disable it in software if you wanted... https://www.crowdsupply.com/interrupt-labs/ovrdrive-usb/updates/a-look-at-our-firmware-and-how-to-modify-it

I use a USB BD-R burner and disks for this. I don’t have a solution for Bad USB protection though unfortunately.

Not saying it's in your budget, but the iodd drives are very useful and are able to run multiple vdisks at once.

And they do have read only modes you can put them into.