I am looking for a power amplifier for the 137 and 472 KHz bands, similar to the DEM for 144 MHz, see http://www.downeastmicrowave.com/product-p/2mldpa-75.htm
Not many commercial option that I am aware of.
I have used a 100W amp from a junked HF transceiver with 50W out for a mW in at 475KHz. Home made filters.
I am cloning a SpilsburyNDB http://www.matthewkendall.com/electronics/lwx-100a as I have the bits.
0.1MHz to 1MHz. Some are sometimes available on eBay.
Minicircuits may have a brick amp but will not be cheap and you will still need filters and switching.