I have an extra US redemption code for Decathlon 2012 (DSiWare), and I'll trade it for just about any Gameboy VC or DSiWare title. Below is a list of all the Gameboy VC and DSiWare games I already have.

Gameboy/Color VC
1) The Legend of Zelda: Link's Awakening DX
2) Super Mario Land
3) Super Mario Land 3: Wario Land
4) Gargoyle's Quest
5) Metroid II: Return of Samus
6) Donkey Kong

DSiWare
1) Mario vs. Donkey Kong: Minis March Again!
2) 99 Seconds
3) Candle Route


Side Note: We can trade our redemption codes via PM. If you would like to be 3DS friends, we could also trade codes via Swap Note.

How do you get redemption codes? Can you get a code if you already have the game downloaded?

I simply sent an inquiry to Cinemax Games about a free redemption code for Decathlon 2012, and they sent one to me in an email.
I haven't found anyone to trade with yet, though. I was hoping to find someone who would trade an extra Golf (Gameboy VC) redemption code. I guess there is less demand for certain sports games than there is for Golf (Gameboy VC).