Here's a vector image:
http://en.wikipedia.org/wiki/File:Municipal_Flag_of_Chicago.svg (not a file, but a webpage).
Vector = infinite resolution.
I actually have been in touch with Mike over the last month and he couldn't find his original file or higher resolution image so I was curious if anyone had anything similar they had also made. I was quoted by one of the companies to digitally clean it up but was more than I wanted to pay. The highest resolution image online is the exact one that "da' Square Wheelman" posted which is sadly only 96 dpi and wouldn't look good blown up to a 3'x5' size. Guess this will have to wait for now.
Here's a link to a hi-res PDF.