I print on all sorts of paper that I have around, and cut it to suit my needs - I have never gone out and bought 'postcard paper'.

My originating postal system has no special postcard rate, so I could send them in an an envelope, but where would the fun be in that?
It does allow that the largest standard sized letter here in Canada is 4.7x7", so I print onto 5x7, and then trim them to suit. Somes my postcards are printed with borders, and others printed bled to the edge of the sheet (especially since I put in a vaccuum easel to deal with curve of old FB papers).