While going about how to register for the AP exam, people tend to put off finding a place where they can take the them until the last minute--and that's the worst thing you could do, especially in a place like New York! Unlike the SAT or ACT, there is no online AP test registration and there are no test centers set up specifically for these exams. It’s left up to each school to allow (or deny) students who wish to test with them. I always recommend the following:
1. Call the College Board directly to get a list of schools that allow outside students to test with them. This is not a list of all schools that will offer AP exams, just those that are willing to say that they'll allow non-enrolled students to test with them. Call early! They generally have a cap on how many outside students they can/will allow, and these spaces fill up fast. In addition, the AP exam registration deadline is typically at the end of March—for schools to order AP exams from the College Board. With spring break and internal deadlines, this means that the beginning of March is when many schools have their deadlines.
2. Research the schools in your area which offer AP courses in the subjects in which your child is interested. As I mentioned, not every school who offers AP courses will list themselves as being open to outside students in order to avoid being inundated with inquiries; however, many schools will allow outside students if they have a few open spaces (the cost of each exam includes a small fee to the school which helps to defray costs, thus providing at least a little incentive for them to allow outside students). Again, call early! It's hard to know which won't allow outside students at all and which just don't want to advertise it; the only way to know is to call, and make sure you get the school's AP Coordinator on the phone to get an actual confirmation one way or the other.
AP exams are an investment. But this investment only makes sense if you have a real expectation that you'll either greatly improve your child's college admissions prospects or score well enough to achieve college credit. (Parents and students also need to be aware of what their preferred schools will and won't accept with regards to AP credits--not all schools treat AP credits equally. But that's another story.) Good luck!