I've never found a source for the black stuff. I've tried buying it from TPS but they just cut copper pipe off a roll
Instead of replacing them, I remove the black coating where it is failing, clean the corrosion off the pipes, and paint them using something tough like POR15. If the union is stuck on the end of the pipe (because the corrosion has wedged the plastic coating against the inside of the union then I will remove the hose by turning the caliper (allowing the union to stay put) and then heat the union with a micro-blowtorch until the plastic bubbles out. That will allow the union to be loosened so that it can be moved away from the flare, and then just clean and paint again.
The only place this doesn't work is on the ABS unit since you can't turn it round if the union is stuck.