It's unlikely that you'll find responsible breeders through classified ads, but I suppose it's possible. Your best bet is to contact your local boxer club and perhaps attend a show. I'm really not sure what to tell you as far as cost - it's not something I'm really familiar with, and costs vary greatly in different areas. I'm guessing that you'll probably pay a bit more from a responsible breeder, but that's not always the case - prices vary a lot with BYBs - some really are after money and are selling them for the same price as good breeders because so many consumers don't really know the difference. Keep in mind that spending more money to get a well-bred dog is worth it because you increase the odds of having a very healthy pet, and that saves you money in the long run on vet expenses.
It can be very tempting to buy from a BYB because the puppies are so cute, and not all BYB's are terrible people - many are otherwise great people who really love their dogs and take good care of them, but they just don't know enough about breeding, so they aren't doing it responsibly. Even that type of BYB is a detriment to the boxer breed, and the only way to stop them is for people to stop buying from them.
If the initial cost is an issue, you could also consider a rescue. Adoption fees are very reasonable. It's unusual for rescues to have baby puppies, but young dogs are pretty common, and if you are willing to consider an older dog, they are truly wonderful pets!