How do you get chansey in Pokemon Crystal?

Chansey is rarely found on route 14, 15 and 16. Keep on searching that area until you find one. Itll take around 15 to a 30 minutes to find one. Then, try using a Pokemon that's level is close to the chansey's and wear down his HP until it is red. Then use a ultra ball if you have one to catch it.(status problems like paralyze and sleep might help too.)

I think it might be one route 13 too. But you got around 1% chance to find it, so be patient.