All the Big Motorcycle events, Datona Bike week, Myrtle Beach, The Big one in Idaho, Christmass Toy runs in all different Cities, nights out at bars, even right here in Nashville, lots of it. Sometimes just in the parks on a weekend you see it. Not necessarly a Big event. Lots of folks wear it.
Gee I was going to mention those events. You might also find lots of people wearing leather at Halstead Days in Chicago. Not sure if Market days is the same event. There is always something going on in Chicago.
Another big biker event is the "Blessing of the Bikes." in Baldwin Michigan. They usually have over 30,000 bikers in attendance. The only bad thing is the event is held in the middle of summer, so all those people in the scorching sun in leather doesn't smell all that great.
If you are looking for leather gear, you will certainly find it there. But, it's usually not cheap, as the vendors charge a lot more than what some of the items are worth. But, you might find a good deal. As the more vendors there are, the more they have to compete on prices.