If you're looking to purchase clickers (also known as response systems or student response systems), there are a variety of places that you can shop.
First, you can look at a local electronic store, or any big box retailer like Walmart or Target. Look through their home electronics or tech departments. Depending on how in-depth you're looking to go, you may find basic clickers here that allow students to answer simple questions in a classroom setting.
Another option is to search online. You can look at websites such as Amazon or eBay for the type of clicker you're looking for. Be sure to review product descriptions and vendor ratings before making a purchase.
Finally, you can contact an education technology vendor directly and inquire about their clicker solutions. Many have solutions tailored to the classroom, with additional features such as remote exams, polling and presentation tools, and more.
In the end, it all depends on how much you want to spend and the features you want your clickers to have. Whichever option you decide to go with, it's a great investment in the education technology industry.