The only locations I know of are Dixie Gun Works, Track of the Wolf and numrich gun parts. They may have what you need or be able to cross reference parts that will fit.
Want ad, gun shop, gun show, egunparts
100-400
1:28
60-300
You can find parts for your 50 cal CVA Apollo muzzleloader at various online retailers specializing in muzzleloader supplies, such as MidwayUSA or Brownells. Additionally, CVA's official website may offer replacement parts and accessories. Local gun shops or sporting goods stores that carry muzzleloading equipment might also have parts available. It's a good idea to check forums and community groups focused on muzzleloading for specific recommendations as well.
Try the forums on the gun broker website, there is bound to be someone who has one that can assist you, also try the high road forum. I placed the addresses in the links section here.
100-300 USD
Depends on condition and any accessories. Value range from 100-200.
Contact the maker of your rifle for their instructions. It will depend on bullet weight. Assuming you mean a .50 cal MUZZLELOADER- and not a .50 Barrett- between 75 and 90 grains of ffG black powder is an average load for a patched ball.
Try egunparts
I bought two of these about five years ago, and paid $69.95 a piece for them at Bass-Pro, but they only sold them for one year.
Value will depend on the exact model, Interest in the traditional style muzzle loaders seems to have dropped in favor of the in-line 209 primed rifles. Figure about $250-300. I have a TC Renegade in .50 cal- great shooting rifles.