There are websites & apps available for mobile devices that can banish the need to purchase separate “clickers”.
Poll Everywhere, for example, is a webservice that can be set up to allow votes to be registered via text messaging or the web. For more information check out http://polleverywhere.com and if you’d like to test it out, go to http://www.polleverywhere.com/multiple_choice_polls/LTExNTQ4MDEyOTc and text in your vote.
eClicker and eClicker Host are apps available for “i”Devices that also allow you to set up polls. Those without iDevices can cast votes via the web. For more information, visit the iTunes App Store, or the app website at http://www.eclicker.com/