where can I get the right converter to plug a 300W 120V US fan heater into a UK socket?

[tabbypurr]

not at all, a 1kW IR heater provides 1kW of heat, enough for a small bathroom. Some are lower power of course.

NT

[Johnny B Good]

It's the absence of evaporative cooling from the forced convection of a fan heater that makes the 'Infra red' heater a better fit for bathroom use. Even when the draught of air from the fan heater is very warm, the evaporative effect can still chill wet skin unless the air in the bathroom is virtually 100% humid.