There is no way other than trading/migrating. Using the Safari Zone you can get a Shelgon at level 46. That means just four more levels are needed to evolve it into a Salamence. Unfortunately, they appear after 70 days with 21 peak objects in the wetlands, so unless you've already got a good start on this, you won't be getting that Shelgon anytime soon. And Bagon are even worse than that to get, 40 more days than Shelgon and more objects of different types needed. Your best bet would be to get Bagon in R/S/E in Meteor Falls and then migrate it to your SoulSilver. Unless you happen to have Pokemon XD and the right tools to tranfer Pokemon from a GameCube game to a GBA game. In XD you can get a Shadow Salamence, then purify it, trade it to a Gen III game, then migrate it over to SoulSilver.