I can't find anything about apartment locators either, which is strange because I would have guessed that we had some given the huge number of apartments in this city.
But I found some other things that might help, here is a page from the University about finding an apartment (for grad students):
Housing - Graduate & Professional Admissions - The University of Iowa
There is a link on that page to a pretty long list of rentals:
Off-Campus Housing Service - The University of Iowa - Iowa City, Iowa
The Iowa City Press-Citizen's search tool might also be useful:
Johnson County*Iowa Apartment Rental | Apartments for Rent in Johnson County*Iowa
And, yeah, Craigslist would also be a good place to look:
iowa city apts/housing for rent classifieds - craigslist
To answer your question, I'd look at all of those and pick out the ones that are of interest to you, and then go check those out. Different landlords advertise in different places, it seems, so looking at different sources is the best way to make sure you are seeing all of the available apartments in the area.