I am not completely sure but I have heard there is a rare stone around the place you first met Prof. Rowan,and you have to use your Dowsing App. on the Poketch and if you use it in a patch of grass one of the starters will come to you. The only other way is to trade with another game.
Chat with our AI personalities
You can get another starter if you get all the sinnoh pokemon. Proffeser rowan will ofer a johto starter.
you cant in platinum but you could trade from Diamond and Pearl or another Platinum.
First is 567pokechhskshks
There is no "worst" starter, it's all about your personal preference. They are all good starters. The most challenging is turtwig.
You either choose a monferno for your starter. Or if you choose a turtwig as your starter your rival will have monferno.And you can find it in south on route 212 and ranger jeffery will have it. That is all the way you can see monferno in pokemon platinum.