http://shopsmart.networkwcs.com/sloanex/index.cgi/GRAIN_HANDLING/GRAIN_GATES?SPD=zwg08509682r1oE

Sloan Express, at the above link, lists several sizes of grain gates.  They did not show an image, but you could call and affirm this is what you want.

Rural King, our local farm supply store chain, used to carry a wide range of grain gates like you want.  I bought several, used them on grain bin doors for easy access.  I haven't looked in the store for them lately, and don't remember running across them while meandering the stores in search of other items.  I searched their database on line and they did not list them, but there website leaves a lot to be desired.  they have a lot of things in the store I cannot find on the website. 

I would be glad to shop for you locally, if you cannot find one elsewhere.  I have other items I am holding for a certain individual we both know in east Kansas, we might find a cheap truck that would bring one part-way.