i searched for months and managed to find a couple of parts-bikes, and although i do have spare parts for my bike i dont have enough stuff to sell anything. Thousands of these old bikes are laying around in junkyards and salvage yards.. just gotta find one.
Cables are cables and whatever you can find or make that is long enough will do the job. The end of that brake cable is has a threaded thing for the adjustment screw. A barrel fits the arm cutout and the adjustment nut is at the end.. This cable is just like the cable on another Honda bike i have. Some research at the parts counter might find you a cable from a Honda bike that will cross over to yours. Many PA50 parts are still available through Honda.
Shoes may also be available through Honda.. give them a call.. Replacement is easy.
It may be possible to re-tread the brakes you already have. I've not done it but i remember threads where people have said they have done it. You might get answers if you start a new thread with "Brake Relining" as the topic.