Does anyone know where I can get additional power supply cable assemblies from ? Without having to keep buying modems ?
Have you asked your Distributor 
Molex do Microfit cable assemblies (both 4- and 6-way):
6-way - Molex Part No: 79516-1101
molex.com/molex/products/dat … Lang=en-US
Available, eg, from Mouser:
uk.mouser.com/ProductDetail/Mole … Nb3iD8XUpz
And, of course, Microfit is a standard part carried by many distributors - so you could make your own…