I adopted my little guy from CraigsList. You can check local reptile shows or local pet stores. I would generally be cautious with those suggestions - you run the risk of picking up a sick animal from either a disreputable breeder or from a place that doesn't take proper care of chameleons. It comes up on the forum all the time.
I would recommend ordering one from someone with a good reputation in the cham world. This forum has a number of sponsors that offer Jackson's. Shipping adds a little cost, but they usually have some sort of guarantee on the animal.
You can always check the Fauna Classifieds Board of Inquiry (Google it) to see if anyone has complimented or complained about who you are thinking about buying from. It's entertaining reading, too.
Before you even get your cham, make sure you have a local exotics vets that treats chameleons and some emergency money set aside. It's very rough to not have a vet in case of emergencies. By the time your little guy looks sick, he may actually be very ill and need treatment asap. Fecal checks can run about $20, visits for eye or mouth problems can be $50-$100, and serious issues can easily run $100+.
Don't let that scare you, tho. I'm madly in love with my cham - such a rewarding pet to have!