Here's my suggestion...
Contact Valu-Pack/Clipper/local ad runs and run your ads through here
Get a listing of local mom's clubs (MOMS International, multiples clubs, Mother-to-Mother, MOPS to name a few) and see if you can advertise in their newsletter. I am on the board for my MOM's Club and we will probably start accepting non-member advertising shortly after the first of the year. It's typically very cheap and a great audience.
Contact local newspapers. First see if you can do a Press Release (for free) and if so/not, place an ad in the paper. Many papers have special childcare sections.
Contact local parenting magazines and advertise here - ex. Chicago Parent, etc.
You could also simple take out adds in Trib, Sun-Times
You could also promote probably through Craigslist
Check out nanny sites/etc and see if you can advertise through them - ex. sittercity, care.com. Most likely you won't since it's an individual basis but you never know.
Contact local churches/pre-schools and see if you can advertise through them.
Contact local businesses for their vendor fairs and set up a booth. For example - Babies R' Us typically has once a month vendor shows at the store (I know Orland does) and you could set up a booth there.
Get onto Message Boards and advertise through here... Yahoo Groups, Meetup.com, etc.
That's what's coming to my head at the moment.
Congrats on the great start! Wow - 400 openings! How many providers do you have? Please PM with your information... you never know when I might need care. Thx