Finding Level Balls
You cannot buy Level Balls. The only way to acquire Level Balls is to have Kurt in Azalea Town make them for you. You need to give him a Red Apricorn for each Level Ball he makes. You can find Red Apricorns on trees in Fuchsia City, Route 37 and Route 44. You can also purchase Red Apricorns for 200 Athlete points at the athlete shop in the stadium in Goldenrod City on Monday, Friday or Sunday. It takes Kurt 24 game-hours to make the Level Balls after you give him the Red Apricorns.
Level Ball Capture Rate
The capture rate for a Poke Ball is multiplied to the base capture rate for a wild Pokemon to see how likely it can escape a capture attempt. The Level Ball capture rate is one, if used on a wild Pokemon of equal or higher level to your current Pokemon's level. This is the same as a normal Poke Ball. If your current Pokemon is a higher level than the wild Pokemon, the capture rate for the Level Ball is two. This is the same as an Ultra Ball. The capture rate is four at double the wild Pokemon's level. This is as effective as most specialized Poke Balls, but without the target restrictions. The capture rate is eight at quadruple the wild Pokemon's level. The Love Ball is the only other type of Poke Ball capable of reaching a capture rate of eight.
Level 100 Pokemon
The Level Ball is the most effective when used with a Pokemon at the level cap of 100. No level 100 wild Pokemon exists, so you can never have a capture rate of one when using it. Ho-Oh and Mew Two, both level 70, are the only wild Pokemon with a high-enough level for the capture rate to be two. Any Pokemon level 50 or below has a capture rate of at least four with the Level Ball. This includes all of the legendary Pokemon, excluding the two level 70 ones. Any Pokemon level 25 or below has a capture rate of eight. This is high enough that you are almost guaranteed to capture one, even if it is at full health.
Usefulness
Despite being able to generate one of the best catch rates of any type of Poke Balls, Level Balls are not very useful in "Pokemon: SoulSilver." You can buy Ultra Balls from stores, so there is no point to use Level Balls unless you are more than double the level of the wild Pokemon you are attempting to capture. Most players should not have a Pokemon with a high-enough level for these to be useful when attempting to capture the legendary Pokemon when they are encountered. The only real use for a Level Ball is for capturing Pokemon that are less than a quarter your current level. At this point, almost any attack you perform would kill the wild Pokemon in one hit, but most types of Poke Ball work very unreliably on a Pokemon at full health.