The Level 99 Shiny Magikarp holding a Rare Candy?
Possibly never. As of now it's an event only at the Japanese Pokemon Centre in Nagoya.
Americans have a decent event on now. Via Global Link on Black, White, Black 2 and White 2 you can add a Level 48 Garchomp into the game which is catchable only in the Entree Forest.
Ends on March 21st.
Europeans have the same Garchomp event as the Americans.
Once again, ends on March 21st.
I'll try and post every so often when there are events on in the UK or US.