sorry to burst your bubble, but ariados can't learn shadow ball.
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It can learn Shadow Ball.
misdreavus you can get this from Pokemon leafgreen. Or shuppet/banette can learn it and even duskull/dusclops. gastly and all evolves can to
level 45 in fire red leaf green
TM 30 is Shadow Ball
You can obtain the TM Shadow Ball when you defeat Morty in Ecruteak.