There are several avenues you can take, i.e., search Craigslist and other local classifieds, work with a Realtor who can approach long-market time homes on the MLS to see if the sellers would be willing to strike a rent-to-own deal.
I have to advise you that you should be very careful about what type of lease/purchase situation you get yourself into. There are 2 types: Lease Option and Lease Purchase. They both will require to you pay above-market rent so a portion of your monthly payment can go towards your down payment, they both will require some initial money upfront (i.e., 3 months rent), but the lease OPTION is just that: You have an OPTION to purchase. Per your agreement, if you choose not to exercise the option to purchase by the date set forth in your agreement, you just lose your initial deposit money but are not subject to any other penalties.
With a lease PURCHASE, you actually enter into a lease and purchase agreement at the same time, with a sales price agreed-upon upfront and a closing date set for sometime in the future to actually consummate the purchase. With this type of rent-to-own agreement, you had better make sure you are qualified to obtain a loan when the time comes, because should you back out, you will be in default.
Either way, be aware that should you consummate the purchase whether through a lease option or lease purchase, make sure you work out the numbers in advance. In Chicago, rents are usually cheaper than ownership, since they do not take into account taxes and assessments and other ownership operating expenses. Know that when the lease part of your agreement expires, it is very likely your monthly payment will go up. Be sure you qualify for it and are capable of paying it by proactively speaking to a mortage broker prior to getting into any agreement.
And one last word of advice: Make sure you have an experienced real estate attorney drafting the agreements/contract for you, or at the very least reviewing the documents drafted by the seller/owner.