You can get replacement cables from J and R Trailer Repair Home - JR REPAIR
or from Midget RV & Trailer Specialists
I've read that some folks have found cable, cheaper, at various places, but you need special tools to 'make' the ends, etc. There's nothing special about the Hi-Lo cables, they're not spec'd just for Hi-Lo.
As for instructions, there are some detailed instructions on this forum. Make friends with the search function. It's not terrible easy, but with some digging around you can find it.