You can find it from Route 45 and Mt. Silver. For more info click link below.
You can find a Steelix in Route 47, Cliff Cave, Mt. Silver, or you can trade any Pokemon with Jasmine.
You can get it from a trade, the GTS, or find it possibly in the waters between the Fight Area, the Resort Area, and the Rest Area. You can find tentacool by just surfing just about anywhere. Or, using a good rod or old rod.
Poliwag can be caught by using any kind of rod in Route 43 (Pokémon Heart Gold / Soul Silver)
Fight trainers, some you need to transfer from other games such as, Pokemon emerald, Pokemon Leafgreen and more. The battle tower is a good place to see transfered pokemon. You have to travel all around to see all the pokemon. It takes time
Use Surf on Route 205 and you'll easily find a Tentacool.
In RSE you can find Tentacool on Route 103.
There are three Pokemon you can catch without a rod on Route 132 (by surfing): Tentacool, Pelipper and Wingull.
I don't know where you can find a trainer with Tentacool, but you can easily find it surfing on Route 205.
In Route 38.
in Pokemon soul silver you can find it at route 36 or route 35
You can find a Rapidash on: Route 22, Route 28, Mt. Silver
Route 34.
route 14
at route 28 at night
You can find it on route 16, 17 and 28
Route 5,6,24,25,31,32,44 is where you find bellsprout