We want to install a steam shower cubicle in an outbuilding which only has mains cold water supply. What are our options for a steam shower... is a power shower option available?

A cold only fed is not an option for steam showers I'm afraid. This is because there would be no showering option. A cold only fed would need an electric shower to be inside the cabin, electric showers are not able to be installed in environments that do not allow the condensation (steam) to escape as this is a higher 'IP' rating than the electric shower lawfully allows