After weeks, of searching and re-challenging every trainer in platinum I can tell you that there is no-one with any of the original starters (bulbasaur, charmander or squirtle) The battle tower does have them, however, they won't be recorded in your pokedex.
I know, it's annoying, I really wanted them though the sad truth is you will have to trade with a friend or bring them through leaf green or fire red.
You have to migrate it from LeafGreen or FireRed.
You have to go at eterna forest and keep walking until you fin bulbasaur.
You have 2 migrate r I can trade you 1
I'm not sure if a trainer has it in platinum there might ,but there is in the battle tower
You can't record it as seen in your pokedex normally. You have to transfer from Fire Red or Leaf Green or trade with someone else to record it
You catch bulbasaur when you get your natinol dex on route 224 hony tree and just wait
How has it in the battle tower? I NEED to see the starters so I can breed them and give sum to my dad to help him with the national dex :)
You will run into a Trainer on the way to gold city I forget the name
Well Another Way is To Have Action Replay for DS and use the cheat for Capture other trainers pokemon. The person you can catch it from is near Floroma town and Eterna City. The Code is 94800004 00000000 92249cf4 00002019 12249cf4 0000e00a d2000000 00000000. I can proove it works because I caught a Linnone(The Evelution of Furret) in Platinum from a trainer by fight area. Hope it works for you!