I know that Van Voorst Equipment in Sioux Center sells auto sort equipment and should have one way gates of some sort.

712-722-1462

Another option is Lange Ag Systems from Willmar, MN. They sell Schick Auto sort scales and have one way gates.  That is where mine came from. They work nice but they are kinda loud and the locking feature isn't real handy. Also they are mild steel so they will rust out at some point in time.

http://www.langeagsystems.com/sorting_scales.html 

There was a really nice SS one way gate at the Iowa Pork Congress this past year that had a very very nice locking feature and was very quiet also. If I can find the brochure for the company I'll post it but I think I may have thrown it out.