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.
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You trade it with a girl on route 14 in crystal for a chansey, which can also be found in route 14
Chansey is rarely found on route 14, 15 and 16. Keep on searching that area until you find one. I think it might be available at all times.
Happiny evolves into Chansey. It evolves with a oval stone in the day
Chansey > level up with max happiness > Blissey
One needs to increase Chansey's happiness to its maximum.