Where can these plugs be obtained, are they expensive, and do they
really save that amount of energy?

I have no idea what it is, but you can be sure that it doesn't work. The
fridge manufacturers go to a lot of trouble to get good energy efficiency so
that they can put 'A' stickers on the things in the showrooms. If there
really was a 50p device to save 20% of the energy for free, it would already
be incorporated into the design.

Christian.