Actually, they are replaceable. Blasters and banshees use the same ball joints. There is nowhere that I know of that sells the joints, but I'm sure they are available somewhere. I highly doubt Yamaha makes them, they are probably made by a vendor. Back when I was racing Blasters, I kept an eye out for bent or broken Blaster and Banshee a-arms...usually you can get them free or for a couple bucks.
To actually replace the joints, grind off the metal plate on the back side of the joint. Remove the ball joint which is actually just a stud with a ball on it inside of a plastic cup. This assembly will just fall out if the plate is removed completely. Then just pop the good joint in there, make a new cover plate out of steel. and weld it on.
This does require a little bit of work, but its not too bad. The main reason I did this was because I had my stock arms extended so I couldn't just pop another a-arm on there.
