You can find a lot of different film and TV scripts on line. An excellent way to practice your American Accent is to have the script while you are watching the show/movie. That way, you can underline difficult words and sounds and make various marks for melody and intonation.
Pay close attention to how the actors are speaking. Then try to read the lines the same way.
Warning! Certain types of films have a lot of slang and bad words. Be careful about who you imitate. Is the character a gangster or a lawyer? Of course, the language they use would be very different.