The fins used in the video are Slim Fins, which are sold at:
http://www.forcefin.com/FF_PAGES/FF_...im_product.htm
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.
Bob