YOu're forgetting, the Celebi Glitch in R/S. That Celebi can be in any ball, as it is wild, but due to the still-unconfirmed rumors that it can flee, it's most likely to be in a Master Ball, like mine.
For those of you that don't know, the Celebi Glitch is an error in the programming of early U.S. copies of Ruby and Sapphire. So far, nobody even knows if it is in the Japanese versions. Anyway, it allows an extremely slim chance of finding a wild Celebi on Routes 121 and 123. It is much rarer than a shiny.
http://bulbapedia.bulbagarden.net/wiki/User:Missingno._Master/Celebi_Glitch
^ this will tell you more. The reason the page is in my own userspace is because all the controversy over the glitch has caused the Bulbapedia staff to believe that the glitch doesn't exist, like so many other people do. Even has the odds of the Celebi appearing; 1 in 268,435,456.