There's very little merchandise for the Avalon Code DS game.
Artbook[]
In the first edition of the game, japanese costumers also got the artbook of the game.
Drama CD[]
Additionally to the artbook, the customers of the first edition of the game got a drama CD.
Phone Cards[]
In Japan, you could get some limited phone cards.
Figurine[]
At the Wonder Festival 2012 the sculptor Yoshihiro Sato made a figurine of Tia, the heroine of the game. It could be bought for ¥5000.