Here's the flyer!
Chicagoland Orchid Festival
I believe this is separate from the Botanic Garden's show (which was in March). But there are international and national vendors at both Hausermann's and Natt's.
Side note, I learned last winter/spring that attending shows before outdoor temperatures are, well, temperate, is a dicey proposition. I lost a few orchids just from transitioning them to and from the car. September is fine for temperatures, and also a lot of greenhouses have sales to move their inventory!