In Chicago, since source of income is a protected class, almost all rental are in theory potential Section 8 rentals. In the outlying suburbs, it is a choice the owner can make prior to going onto the market.
The bigger challenge is to find a building owner and/or manager who will work with the credit issues most (but not all) Section 8 voucher holders have.
The best thing to do is work with a broker or leasing agent that has experience in working with Section 8. You can also search Craigslist and in the search criteria make sure you have your neighborhood and place Section 8. Some advertiser only put in Sec 8.