You specified a frequency of 134.5kHz. If that is correct then it is unlikely that an iron cored transformer will meet your needs. You will probably need a ferrite core unit and at those sort of frequencies you should be able to wind your own using shellac coated (or equivalent) copper wire. Or alternatively buy something like one of these http://uk.rs-online.com/web/p/telecom-transformers/0196369/. From its data sheet you'll see it offers high frequency bandwidth and high voltage isolation. Whilst its ratio is 1:1 rather than the 5:1 you desire, you can use a simple resistor divider on the output stage to get your desired ratio.