300VDC to 24VDC voltage converter - Cheapest implementation
How many lamps are you running simultaneously? I ask because dropping the mains to somewhere in the 24-48V region makes your control problem easier, and if for example only say 20 of those lamps are on at any one time then you are into the place where cheap surplus 'telecom rectifiers' are a thing. 

A 2kW 48V telecom rectifier is not expensive on the surplus market, and 48V would increase your duty cycle in a PWM arrangement significantly, possibly even allowing simple PWM (Watch the difference between RMS and average here) without much in the way of inductors. 

TH lamps have significant thermal time constants so flicker is negligible by the time you hit even a 1kHz switching rate.