The fins used in the video are Slim Fins, which are sold at:
Be aware, though, that the youtube video you pointed to is an old Total Immersion training program. (I'm also not sure the video was posted with TI's permission, and if not, it is a violation of TI's copyright.)
The current program is outlined in the Betterfly DVD and in the book Extraordinary Swimming for Every Body
, both of which are available in the Store section of this site. The new program doesn't depend on fins, which is a definite advantage, because you can't wear fins when you're in a swim meet. I originally learned butterfly using the old TI program, and it produced a terrific butterfly, but I had a lot of trouble breaking away from the use of fins once I learned it.