I'm looking at ticket websites, and it appears that there are sporadic tickets available on Stubhub, VividSeats and Seatgeak. Right now the only tickets available on the official KU Ticket Office site is for the four-game "Arrowhead Packs." I'm guessing a lot of KU fans are getting these and trying to sell the ISU game. So far it appears they've only sold out sections between the 30-yard lines in the lower level.
Arrowhead Pack Link
Arrowhead Pack Link