Proper places like Digikey and Mouser will have M.2 connectors nicely sorted by key, height and mounting type. In case you, like me, choose price over convenience by shopping at LCSC, about a ...

And, if you have a simple external adapter that has only a PCIe connector ... is M.2 form-factor, called 1216. It’s solderable and hence not as easily upgradeable, but with a pre-defined pinout ...