Sir here for nerdy questions :) The one Kitty usually wears is
It has been harder to find in the US for some reason, but overall fits the mouth well for a nice tight look. (Rima brand, but some places still sell as a generic penis gag)
Mister-S also had some well made variants of it, but for some reason it is not on their website at the moment. They do still have a nice buckle in the back variant though:
Top-To-Bottom Leathers also has a nice one, just will take some breaking in to wrap around the face well, due to the nice thick leather used.