You can catch it at the Safari Zone in Fuschia City but it is rare. Throw rocks at it and make sure it doesn't flee so you can catch it easier.
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The easiest and only way to get a lucky egg in Pokemon Fire Red is to catch a Chansey and hope it has one. The best place to do this is the Safari Zone.
safari zone
As far as I Know Chansey is not on the Pokemon Sapphire you can trade it from other versions like fire red or leaf green
The item Lucky Egg, that doubles the Exp.Points earned in battle, can be obtained by catching a Chansey in the Safari Zone. You will have to be lucky though, since there is not really that high percentage of getting a lucky egg with the Chansey you catch (and it is not easy to meet a chansey, nor catch it, either...). As a sidenote, I actually got lucky as hell and receive a lucky egg on the third Chansey I ever caught in Firered. If I forgot to mention it, the Chansey will be HOLDING the item, in case you are lucky enought to catch a chansey with it.
chansey does not evolve by level. it evolves by happiness. i hope this helped!