You can get them from any vendor of retail systems and a lot of computer stores. The simplest ones are magnetic stripe readers that behave as though the user was typing the data from the stripe. I had to familiarize myself with them for a project I worked on in the mid-1990s to enable health care providers to read swipe health cards and transfer the contents to a database. The part of the early testing I enjoyed most was swiping all of my boss’s credit cards into a text file.
So they have a legitimate use. It’s too bad people are misusing it to steal.