If I was in the city today, I'd say buy me lunch and I'll give you one of my shirts. I have a couple that were given as gifts that are just a notch to small and have never been worn. Other than that, you'll have to hope for a merch table. I don't think I've ever seen ISU gear for sale in the area.
the alumni club is having a raffle for the pregame party at stanley's and there are a few tshirts included in that i believe. you could always bribe someone to make sure you get one of those, it wouldn't be chicago without dirty politics and under the table bribes.