It depends on what kind of jamming device you need.
But typically, cell phone jammer can only be bought on the online shop.
If you're just trying to insulate your phone, you can buy it on amazon. If you need jamming devices has a variety of functions, so you can only buy them at a professional jamming device store. For example, perfectjammer and jammer-store.
These two stores are the most professional jammer device shops at present. In contrast, perfectjammer has more products and cheaper prices.