The previous answer was a lie. Seedot is VERY rare and according to the PokeDex, they can be found on routes 102, 117 and 120 but I searched and never found one. Luckily a friend of mine bred a Gardevoir for me which gave me a Ralts. And there is a guy in Rustboro City who is willing to give up his Seedot for a Ralts. Look for him in the house to the right of the gym.
I dunno which is more rare, but according to the PokeDex a Ralts can be caught in route 102. Equally I never encountered one. I wish you luck getting one!
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On Route 102
Seedot is a rare Pokemon that normally can't be found unless there is an outbreak of them on certain route so how do you know when an outbreak occurs? Find a TV in a house and check it every day until Pokemon news airs wait until they tell you there is an outbreak of seedot on route whatever go there to catch seedot.
That's the one left of Oldale town, right? Seedot (rare), Lotad, Zigzagoon, Poochyena, and Ralts (rare). Get a Ralts, they're fantastic.
Seedot can be traded with guy in rustboro in the next to the rustboro gym leader. The guy will tell you he will want a ralts for his seedot.
You can find Nuzleaf on Route 114, but it's rare. It might be easier just to level up the Seedot you should have traded for in Rustboro City.