Go to the snowy part of mt coronet and use a super rod in the room before you get the adamant and lustrous orbs and keep fishing until you see a dratini or dragonair
Where Do You Find Dragonair In Pokemon Platinum?
Go to the Waterfall in My. Coronet and use your Super Rod and fish behind the waterfall. This only works after you have the National Dex. Dragonair also only has a 5% chance of coming to you.
You can find Dratini and Dragonair in Mt.Coronet under the waterfall with Super Rod after getting the National Dex. Dragonair evolves into Dragonite at level 55