They show up quite regularly on EBay. Be careful though, some people just rip the degausing coils off a monitor, add a switch and a power plug. This will work, but these can burn up quickly if you hold tham on for very long. Better to get a commercial one. Or go with a tape demagnatizer. They are a little trickier to use, as the magnetic field is more focused, but they do work.
ken