A reliable Pokemon website reports that a level 15 Chansey can be found in the Yellow Safari Zone (on its Pika Cup information page). However, the same website says on its Kanto Safari Zone page that the lowest that can be caught in Yellow is 21, which is too high for the Pika Cup. The N64 Pokemon Stadium allows for the rental of a level 15 Chansey.
So, does anyone know if a level 15 Chansey can be caught in RBY. If so, how? Thank you!
Yep, it is possible. But only on Pokemon Yellow. Area 1 of the Safari Zone houses Level 7 Chansey. Good luck finding them. Only 1% chance of them appearing.