In Japanese, it's called a 口開器 or 開口器, or マウスワイダー.
Here's a good one: http://www.xvideos.com/video7733272/teen_asian_girl_bukkaked_hardcore_and_forced_to_swallow
This site also has a nice selection. If the link doesn't work, a google search of 口開器 should yield some good results.