Try this link for the Illinois Network of Childcare Resource and Referral Agencies:
http://www.inccrra.org
I have not used their services, but I have colleagues who are familiar with it.
In terms of sitters, we pay $10 an hour and only book people who can actually drive to/from our house. It is too tough to get teen sitters to/from their homes to sit for us, as our free time is really limited and if we can score 3 hours away from work at the same time, we would rather the sitter get here his/herself.
Due to severe food allergies for one of our kids, we only book college age and up, since the sitter would need to learn how to use an Epi-Pen, call 911 and handle an ER visit if needed, until we could get to them. We hire teacher-friends who can use the extra $$ in the evenings. Also, we have used a couple of college kids. You can make a flier and post it on the bulliten board of the local college or send to a teacher training school, such as Erikson Instutite in Chicago. (www.Erikson.edu.) They post sitter-wanted fliers and you'd get someone who is at least of grad school age.
Otherwise, my husband and I stagger our schedules and make use of in-laws and grandparents as much as possible.
In a pinch, we hired a sitter who charged $15 an hour. Ouch!
Good lucK!