In the Museum of Soviet Arcade Machines you can buy an authentic post card of Brezshnev or Khrushchev times and send it anywhere in the world. The post box (the old style post box, an authentic one!) is located on the front wall of the building, just next to the entrance. One postcard is 100 Roubles, post stamps for Russia and for abroad included in the price.
If you visit the Museum of Soviet Arcade Machines in Saint Petersburg (the visit is included free of charge if you have a CityPass) you can send an original old postcard to your friends.