Clickers are available at most major bookstores around campus (the University Bookstore and the Alamo II). Remotes can also be purchased used, just like used textbooks, if available at some bookstores.
(note: be careful that you get the correct model)
Clickers are purchased with a perpetual license. Once they are purchased they are ready to be used for as long as a student is in school. Students should be encouraged to keep their clickers as the integration of clickers in classes is expected to increase dramatically over the next few years.