Can you upgrade all the motors to 24V instead, if cost isn't a problem?
That being said, I'm not sure such a motor is actually possible to build. The problem is that higer current leads to more heat and thicker windings to compensate, to some point where there are diminishing returns. In general, stepper motors are specified by acceptable current, not voltage, and you are assumed to raise the voltage until you get to the current you need.
Anyway, if you want a strong 12V motor, you can keep looking. I'm in the US, so I can't help you on locations, but I think all of Kollmorgen, Maxxon, and Faulhaber are actually European based, so I'd be surprised if you couldn't find an assortment of motors at some motion control / factory automation distributor. If they don't have it, it probably doesn't exist.