Unfortunately you can't get the maximum amount of Master Balls without Action Replay on Soul Silver. The only way to get more Master Balls legitimately is to trade them over. There is no real alternative option in game to obtain them. You can't purchase them from a Pokéshop, and the Pokémon lottery in-game has an almost impossible chance of winning one. Even if you do win, that takes you up to two. Some Pokémon are said to hold master balls, so that may be an option, but again it would be very time consuming and mostly unsuccessful.
With the trading, it's generally a laborious process of getting to the point of receiving a master ball, trading it over, then starting a new game on the extra console you're using. It will work but it will, like searching for Pokémon carrying master balls, be very laborious and require a lot of patience.
If this disappoints you then you can concede and use a game hack. 'Action Replay' is a brand of cheat cartridge developed by Datel. They tend to only be available for older consoles, such as the N64, but due to the inherent cartridge nature of GameBoys it has meant that this is still an option for Nintendo users. You must note though, these cartridges, because of their code-breaking, can corrupt and often do corrupt game files, meaning that you could get 999 master balls but you could also then see your game crash and wipe itself after several weeks of dedicated gaming.
With the trading, it's generally a laborious process of getting to the point of receiving a master ball, trading it over, then starting a new game on the extra console you're using. It will work but it will, like searching for Pokémon carrying master balls, be very laborious and require a lot of patience.
If this disappoints you then you can concede and use a game hack. 'Action Replay' is a brand of cheat cartridge developed by Datel. They tend to only be available for older consoles, such as the N64, but due to the inherent cartridge nature of GameBoys it has meant that this is still an option for Nintendo users. You must note though, these cartridges, because of their code-breaking, can corrupt and often do corrupt game files, meaning that you could get 999 master balls but you could also then see your game crash and wipe itself after several weeks of dedicated gaming.