You can only get it at the bookstore and once you buy it, the clicker will be connected via a unique code to the student ID of your D (i.e. you can’t sell it to someone else, a huge PITA). I’m surprised it’s required for Engineering Analysis though, I don’t remember anyone using it in the past- I guess it would be worth double checking with the professor to see if it is truly needed or not (for instance, in my physics and chem class, we used clickers, but the class had a set that we used for that term, so the students did not have to buy their own)