answered question
You should be able to get them all in your pokedex straight after you defeat the champion. If you are missing a couple, make sure you've battled everyone along the way and make sure you have visited the three lake pokemon, at lake valor, acuity and verity.
answered
Very near the end. From the pokemon league, go over the bridge, down the rock climb rocks and then head towards the left without going down the ledge. After a few steps, it should be there. It is also available on route 224; I have two.