Go to the Mikmar website. Mikmar Bit Company
. There are several different ones. They are pretty expensive though and aren't for everyone. I think they come with a video that explains proper use of the bits. Also, take off the nose band if you use one, otherwise it will arrest the action of the bit.
If he is sensitive on his poll, he may not tolerate the gags because they act through poll pressure. That may be something to play around with and figure out before you drop $100 on a bit. :)