There is ONE way to get SEEDOT in Sapphire and I just caught mine. 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 was misled by reading in the web that SEEDOT is not available in Sapphire, but as I went to route 117 I walked in the small patch of grass. I DID NOT FIND JUST 1, but I encountered 2 (yes TWO) SEEDOTs there. I fainted the first one (duh) and I caught the other one! The PokeDex entry still says the area is unknown. But I successfully caught mine there.