I'm not sure if it's the cheapest place or not, but you can get all three at Amazon.com still, it seems. They all have some new listings.
I would recommend Dawn of Sorrow as a personal favorite, though the main game is a little on the short side. You can keep yourself busy forging weapons and collecting more souls if you have OCD like I did. There are also some new modes you can unlock after completing the game once. I really liked the story and characters in the Sorrow games.
Order of Ecclesia was the last DS game, I am not as familiar with it as the other two but a lot of people really liked that one. I notice it has the smallest number of current offers and is the most expensive of the three.
Portrait of Ruin was also pretty good, my second-favorite of the three. It seems to be the most overlooked of the three, but it was an interesting game because you would travel through the castle and go into paintings, which would take you to other areas like pyramids and whatnot. You also control two characters at once, and can switch between them and perform 'dual attacks'.