I found some decent postcards at Gulf Greetings/Hallmark which have locations in Villagio, City Center, and many other locations in Doha. They did not have postage stamps.
Hallmark/Gulf Greetings (3 QR each)
Adham Gift (10 QR for 3 postcards)
On a mission for stamps at the post office, I noticed these post cards available. See my entry on this post office for details: Q-Post General Post Office Building (Head Office)
General Post Office Building (Head Office) in West Bay