For such a large investment it is wise to hire a real estate agent who will find listings that meet your needs and arrange appointments. Advertisements can be found for real estate agents practically anywhere, but make sure that you are comfortable with your agent because a good agent can help you save a lot of money.
Real estate agents can be found in the Yellow Pages, on-line (http://www.readyrealestate.com/; http://www.trulia.com/IL/Chicago/; http://chicago.craigslist.org/rfs/) or through recommendations by friends.
It is also a good idea to research homes on your own to see what you like and don't like, as well as get a better understanding of what different neighborhoods and housing have to offer. Some websites that will help acquaint you with a variety of homes are: http://homes.dailyherald.com/ and http://www.homegain.com/local_real_estate/IL/chicago.html.