Unfortunately not. It is in the game's coding, however it is an unusable item and is not obtainable in the game...
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you use 1 master ball if you use action replay or you can let a Pokemon and trade it with the master ball. Don't listen to the kid on the top.........you cant trade a Pokemon that's holding an item from Soul Silver or any of those games to the white or black version.
An Ultra Ball or a Master Ball. Carry up to 50 ultra balls (and maybe more; Cresselia is a hard-to-catch Pokemon), and weaken the Pokemon as much as you can. Once it's at the weakest you can get it without it fainting, start throwing Pokeballs at it. (If you can, try to paralyze it or put it to sleep. Do not poison it though; it'll lose health and the Pokemon will faint before you can catch it. Also, legendary sometimes will "run" away in a while battle...To make sure that doesn't happen, carry a Pokemon who has a move (such as Mean Look) to prevent it from running) Don't use a Master Ball unless you have to...Master Balls are used for a right-away capture...And they're the rarest of balls. If you can, try to catch the Pokemon with balls you can buy. In rare occasions, a Heavy Ball or Dusk Ball. Cresellia isn't that heavy, but if you don't have any ultra balls (or not enough), use a Heavy Ball to capture it. And, if you try to catch the Pokemon at night, use the Dusk Ball (works best in a cave or at night). Both, un-necessary if you have Ultra Balls.
use a ultra/great ball
You can't get it in Pokemon Black. Use PokeTransfer or Dream World.
On a certain Pokemon that you want to catch and it's hard to catch it with just a Poke Ball, a Great Ball, or an Ultra Ball. I don't know where else you can get Master Balls besides the Silph Co. I usually use the Masterball on legendery Pokemon.