You can make your own post cards - really! It's easy!
Every summer my DDs go off to camp and of course getting lots of letters from home is important. Writing a full sized letter every day is hard - there's not that much news and too much chatting about home might make them too homesick so I hit upon sending them post cards.
Here's a complete Tutorial on Making Post Cards. I like to include quotes on mine - Teen1 did a college course/camp for 2 summers and used them to decorate her dorm room and door. I love them because they are small enough to write just a bit and cheap to mail - it's easy to send one every day. I print them on card stock, scribble on the back, add a stamp and I'm done! You could get them printed out as photos too if you wanted a more polished appearance.
They make terrific party invitations or save the date notices. You could even use them to make advertising postcards (but you'd need commercial permission from the designers first.)
Here are some I made tonight.
Credits: August Shadows by Miriam Lima; I scanned some fortunes and added them
Credit: Teen Spirit by Kim Hill; font is Genius of Crack; the dog is my standard poodle who needs a haircut.
Credit: Say What by Amy Teets; flower and words from OScraps grab bag; stamp from Picnic in the Park by Kim Hill; file tie by Kim Hill; buttons. ribbons and tag by Corina Neilsen; bent frames by Khristy Schmidt