I don't think you can buy Ethers. But you can farm berries. I'm not sure if they're in FR/LG.
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Neither item can be purchased. Normally you can find some throughout the game.
you can't get him on emerald ruby or Sapphire. you have to trade him from Pokemon leaf green or fire red to emerald. on Pokemon fire red and leaf green you get him at the end of island 1.
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mysty is only in Pokemon fire red or leaf green
you can trade with Pokemon leaf green and Pokemon fire red