In Soul silver there are two ways to get a Master ball. One is too get it from prof. Elm after you get all 8 Johto badges, however he only gives you 1, he will not give you another if you beat the Kanto gymes or do anything else. The second way is to match all 5 numbers in the lottery. So no, you can not get another legit master ball without the lottery, the only other way would be to cheat/hack which I strongly advise against.
