Have you done an ebay search for Aircraft inverters...... Avionics transformers.... etc there are often several listed on there.
stuff like this may be of use
RAF Hawker Hunter Aircraft Electrical Invertor Type 206 on eBay, also, Aircraft Parts, Aircraft Aviation, Cars, Parts Vehicles (end time 01-Sep-08 15:02:57 BST)
eBay Motors: Bootstrap Transformer 115 to 26V or 26 to 115V 400Hz (item 300236568209 end time Jul-01-08 21:14:33 PDT)
basically what I am saying is do an ebay search
also there is this which is no doubt to expensive, they do a bigger one too
500W 0-300V AC 45-70Hz 400Hz Power source on eBay, also Power Supplies, Test Equipment, Electrical Test Equipment, Business Industrial (end time 03-Jul-08 10:22:30 BST)&