I'm not sure if this is true of all showcase cinemas, but it certainly is in Peterborough.
You can get lots of free postcards at Showcase Cinemas, there are some at the Ben and Jerry's ice cream counter (about 8)
and then in the main area past the ticket point, (so you have to pay for your movie ticket) there are lots more postcards. (About 10)
Hope this is handy to anyone planning on going to showcase soon- and it still counts as a good freebie!!