I'd like to ask you a couple of questions before I make comments on what you wrote.
-Where are you from?
-Where do you presently live?

I quite agree that hardly anyone speaks formal language. Unfortunately I don't have the opportunity to look at the script of a movie simply because I don't have a clue where I can take it.

I like your very last sentence. I completely agree with this statement. I was never taught by an ESL teacher, so I can't say anything about them. The only thing I can judge by is this forum. I've been a pretty active member for a fairly long time and I don't have anything against 'local' teachers. I guess they're all ESL teachers. They never say that you *must* use formal language. You ask a question, they just answer it. If you want to know more you just ask them for the second time.
What do you mean by casual speech? Do you mean informal expressions/words etc or something else? I think it's very hard to study informal language with a non-native teacher. An English-speaking country would be the best option for learning English. Teachers are more free then. They don't have to spend a lot of time deciding what their students should study first. They can teach both formal and informal language at the same time. When students come out of the classroom they find themselves in the world where English is the only language people understand. Therefore, they learn both informal and formal. I can give you a good example - talking with a counterman (I don't know what you call him) is a good way to learn a few basic fairly informal expressions you never heard of. There are loads of examples I could give to prove that being immersed in the language is the best way to learn it but I don't think there will be many interested in the matter. The best way to learn English is to practice it. The best way to practice your English is to speak it. The best way to speak English is to come to an English-speaking country. That's easy