A function generator with 60Hz sinewave output feeding a high power audio amp capable of at least 300W pure sine output into a 4R load (so music 'power' in excess of 1KW) feeding a 30V 250VA transformer wired backwards as a stepup. Put a 25W 240V incandescent bulb or heating element across the original primary so its damped and doesn't run unloaded, otherwise you risk blowing the amp. Derate by the crest factor or your load, or again you risk blowing the amp.
Other transformer ratios are possible depending on the amp's minimum load impedance and peak output voltage. If all you can find is a >500W per channel (music 'power') stereo amp, you'll get the most out of it by inverting the input signal to one channel (assuming its got a shared PSU for both channels - if its got seperate PSUs you can drive them in phase) and, on its outputs, using two separate transformers stepping up to 120V, with their original primaries in series for a full 240V.
Its not *cheap* unless you can buy, beg or borrow a used amp . . .