I think it is just random =/
CD3 => To Obtain, you need a Shop Code
CD2 => To Obtain, go to your local Tamagotchi Shop and check if Item is in stock. (Note: There is a 50/50 Chance you will get a CD1 instead of CD2 as well.)
CD1 => To Obtain, go to your local Tamagotchi Shop and check if Item is in stock. (Note: There is a 50/50 Chance you will get a CD1 instead of CD2 as well.)
Also as others have mentioned above ^^ there is a CD in TamaTown..but I think its Random, CD2 or CD1
Hope I helped ^_^
Ok thats revised now ^_^