Where do I buy one?

Buy an iClicker clicker at the bookstore (not interWrite or HITT brand).
USING YOUR UCSD ID (PID starts with A) Register it at iClicker so you can collect your clicker points for your grade.

Do I really HAVE to buy a clicker?

Yes. Why? Well, read the web page on lectures and you'll get an idea. But, hey, computer science isn't something you learn by hearing about. You have to try it out for yourself. Clickers will let you do that, in class, in time for me (and your classmates) to help you clear up any confusion you have. By engaging in lecture clicker questions, you will be better prepared for your quizzes, programming assignments (PSAs) and exams.