How do you get a cyndaquil in Pokemon Platinum?

You can either use an Action Replay to get him/her. Or you can trade it from Gold/Silver/Crystal to an Emerald/Ruby/Saphire game using the time machine downstairs in a Pokemon Center. Then once you have it on Emerald/Ruby/Saphire, migrate it to Platinum.

actuall u cant trade to sapphire from cyrstal and the other gams because its not conpatible bealive me i tried and u cant i have a lv 100 fralagator and a lv 100 typhlosin and they cant trade over but if u have the new souls silver ot the other game then u can get a 2 ds or dsi and trade it over to platinum