It all depends on what the machines are, how big the motors are, and how much you want to spend. Options are:
Static Converter--cheap, but will not run or start large motors very well.
Rotary Converter--More expensive, and somewhat noisy. Works well with most loads.
Solid State VFD--installed on each machine, some models can generate 3 phase out from single phase power. Also gives you the ability to vary the motor speed.
Motor Replacement--if the motors are small enough, and standard frames, swapping them for single phase motors is always an option.