Cutting a princess cut would give a smaller face up size for a given depth and so obtaining a princess cut in sapphire may require that you ask around to see if any cutters have rough which would be suitable for a princess cut. You might also find a stone of adequate size for re-cutting in your searching, (if you do this just be careful that the stone is not too color zoned to allow re-cutting without losing color).
Does it have to be a princess cut or will a square cushion suffice? You will have more success finding a square cushion than a princess cut. I've only perhaps seen one or two princess cut blue sapphires.
We cut princess only on brighter tones, like baby pinks or bright blue (then more diamond lookalike).
A high-end deep Kashmir blue we cut only once into princess, on request. It looked great, however, you do not really see the princess cut as such only the color.
From this perspective it is kind of wasteful to cut a deep blue into princess.