You can find Chansey in routes 13, 14, and 15. (One of which is the same route that the girl that wants to trade an Aerodactyl for a Chansey is wandering around.) Chansey are extremely rare, they have like a 1 in 100 chance of appearing. To make Chansey appear a little more often, put a Level 25 Pokémon in the lead spot in your team and use some Repels/Super Repels/Max Repels. They still will be pretty rare, but you will encounter more of them. You just have to be patient with it.
If you are really looking for a challenge, try and get a Lucky Egg, 1 in 67 wild Chansey are holding a Lucky Egg. ^_^