YOU CAN DEFINITELY GET SEEDOT IN SAPPHIRE, I HAVE CONFIRMED THIS MYSELF.
The Pokemon Seedot is available in Sapphire, just not the traditional way.
There is ONE way (and one way ONLY) to get SEEDOT in Sapphire and I caught one. You need to have battled the Pokemon League and completed the game. One of those visits back home to Littleroot Town, make sure to watch your bedroom's Television. If you're extremely lucky, an announcer will be on TV saying that there was a rare sighting of the pokemon Seedot in route 117.
I DID NOT take it serious because everywhere I went it said that Seedot is not available in Sapphire. They are all wrong.
I went to route 117 casually, I walked in the small patch of grass and I encountered 2 (yes TWO) Seedots there. I fainted the first one by mistake and I caught the other one!
The PokeDex entry still says the area is unknown. But I successfully caught mine there.
There might be other rare Pokemon that can only be caught by rare sightings on TV. But that's the only one I can tell you about. Hope this helps clear out confusion.