The GPR is a good rifle. And accuracy with the PRB is very good. You just need to find which patch/powder combo works the best out of your particular rifle. And as for parts, yes, they are readily available. But honestly, with proper care and maintanence, its pretty tough to break anything on a Lyman sidelock. It happens - but rarely.
It appears from you post that you either got a new one, a used oneor are planning on getting one. Good luck with it. I would start out with .015" patches and 80 gr of FFg under the ball. And if you can find them, you can even try some 54 cal felt wads between the powder and PRB.