Where do I find a puppy in illinois?

Try Craigslist, eBay, Backpage. Many pet owners may ask for a little "good faith" money, which is much less than $400. You can also write a "puppy wanted" ad. Some pet owners have too many puppies. If someone asks for it, they may put them up for adoption.

( list your puppy for adoption http://www.digi-go.com/reehome/ )

If $400 is more than you plan on paying for a puppy, how much do you plan on paying for food? And how much do you plan on paying the vet for a visit? Just yesterday I dropped $200 at the vet, for an exam, bloodwork, and medications. You should figure these costs into your plan, as well.

The initial purchase price is the least you will spend on this dog. A Boston terrier will easily live to be 12, so amortized over his lifetime a $400 puppy will cost you $3.33 a month, or about a dime a day.

Edit: In my life, the lower the purchase price of the dog, the more I spent at the vet, by a factor of 10.

Wait a while and try again at the shelter. You do know that owning a dog is very expensive right? $400 for a puppy isn't that much.

Craigslist but make sure you go into detail with them because they might just be selling you some sick puppy. Look for "boston terrier for sale Illinois" on google. If you are looking for a purebred high line of boston terriers they are most likely going to cost like $250-up.

Keep watching the ads. Looks like that is the price so you'll have to wait for someone who can't get rid of their pups to drop the price.

ETA,

I don't understand how some people figure paying more for the puppy will leave you more money to pay for vet bills.

Nowhere did I see you say you couldn't afford $400, just that you weren't planning on spending that much.

If you can't find one, try visiting your local shelter...I'm sure that they will have a dog that you will be dying to take home :)