Cabela's or Dixie Gun Works would be your best bet, or gunborker website.
Depends on what caliber you are comparing it to.
Try gunshops, gun shows, want ads, for sale ads, on line auction
A 209 is a 209 regardless of what it is used for.
Yes and is very worth your while to install it...google Badger Ridge Remington 209 conversion kit. Badger Ridge has the best one, and has lots of blog pages and videos about it.
They sell between $125 and 250 depending on condition caliber and extras (scopes, stocks, 209 primer conversion)
By 870 I assume you mean the Model 870 shotgun? If so, there would be no purpose for that type of conversion. Shotshell primers are of the 209 size. The 209 "conversion" is done with muzzle loaders. The idea is to replace the muzzle loader nipple (that holds the cap) with a conversion unit that uses the 209 shotshell primer to get more positive ignition of the black powder.
Both would work, however a standard 209 should be plenty, and a magnum primer runs the risk of blow-back that could damage you and or the gun.
Question can never be answered as currently written.
The 209 throws a much stronger spark into the powder charge than a no.11 or musket cap.
Buy a conversion kit and follow the instructions. If you can not do that give it to a qualified gunsmith to do.
Please verify with the NY State Game Commission- and the answer seems to be "maybe". NY does not consider an "antique" to be a firearm. The term Antique includes muzzle loading caplock or flintlock rifles and shotguns, and present day reproductions of them. However, a modern, in-line, 209 primer type muzzleloader CANNOT be used by a felon under FEDERAL law. That prohibits a felon from possessing "ammunition"- and ammunition includes 209 shotshell type primers (but not #11 percussion caps). Some state laws permit possessin of a muzzleloader by felons, some do not- so please verify with your state.
The Wolf Magnum is rated for 150 grains maximum charge and designed for use with Pyrodex pellets or similar pellet type powder and 209 shotgun primer ignition.