I've gotten brackets similiar to that from this website. http://www.trafficsigns.com/hardware-posts/
I don't know if they are wholesale or not because someone else purchases them for me.
That's awesome that they don't have pictures associated with their products...
It looks similar to PN73 from The JRS Company. It accepts 1/8" or 3mm thick material. We use it for flag type interior signs.
www.jrscoinc.com
Todd