I can be anywhere, as long as it isn't in an event location. I guess it can be stationary dudes as well, I guess.
You can find the strongest Pokemon at Mt silver but you cant go there until you beat all the gym leaders in Johto beat the eliet four beat all the gym learders in kanto then go talk to Professor Oak in pallet town and he will give you access to mt silver in mt silver you can find Moltres.
You can catch some Pokemon over level 50 there but the most of them is in 40.
There are quite a few places like Jar Jar above said, so here's an easy to navigate list you can use to find all the locations with Pokemon Level 50 and above in minutes.
Go to the Johnto tab because that's the region you want.
Hope I helped!