Take a look here http://www.jammer-store.com/ for a blog post called How To Make Your Cell Phone Jammer: DIY Guide.
Be aware that a jammer may be illegal where you are due to public safety concerns. In the USA for example, from the Federal Communications Commission:
"In recent years, the number of websites offering “cell jammers” or similar devices designed to block communications and create a “quiet zone” in vehicles, schools, theaters, restaurants, and other places has increased substantially. While these devices are marketed under different names, such as signal blockers, GPS jammers, or text stoppers, they have the same purpose. We remind and warn consumers that it is a violation of federal law to use a cell jammer or similar devices that intentionally block, jam, or interfere with authorized radio communications such as cell phones, police radar, GPS, and Wi-Fi. Despite some marketers’ claims, consumers cannot legally use jammers within the United States, nor can retailers lawfully sell them."