There's a load of people out there offering mediumship development for established mediums.
http://www.spiritualistassociation.org. ... shops.htmlhttp://www.arthurfindlaycollege.org/http://www.ershamstar.co.uk/But before you pay your money to these people, find out who the tutors are and then go and see them on platform. Suss them out, so to speak.
There's a few more links I could post, but mediumship courses are a minefield of charlatans. I am under no illusions in that respect. You need to find the right people for you, teaching what you want to learn.
Perhaps sit down and honestly appraise your mediumship. Go on a course that addresses a weakness you think you may have.
Don't be frightened to tell these people that you are advanced, if you are already a working medium. There's nothing so disheartening as sitting on a course that is going over old ground. You need to be stretched, so be honest about you ability and you will reap the rewards.