there is also a third option, in addition to bought and homemade, you can buy one and alter it!
The way I make my postcards is I have a template (I made it from a bought postcard that I received). I use the template to cut out a piece of cardboard (like the kind cereal boxes are made out of. I cover one side in white paper - this will be the side I write on. The other side I decorate by collaging with paper or fabric. And sometimes I use rubber stamps. I've also thought of trying watercolor on watercolor paper (probably 140 lb), but haven't done it yet.