I'm a mobile app developer. I code for Android and iOS on Mac OS.
I don't want to buy another computer just to code for Ubuntu Mobile.
Will the Ubuntu Mobile SDK be available for Mac OS or Windows?
There is no SDK preview available for other platforms other than Ubuntu at this point.
However, you can either:
The first should be relatively trivial to set up, whereas the second should in theory work, but it's not been tested and obviously not supported.