How do I get Seedot in Pokemon Diamond?

I would like to try and get Seedot on my Pokemon Diamond game. Can I find it on Routes 203, 204 or Eterna Forest? or where can I find it at? I already have the National Dex. But, do I have to have Pokemon Ruby to get Seedot or is there any other way to get it on to Diamond without migrating it, because I don't have Ruby I only have Sapphire. So, I'm wondering if there's another way and if I can find it in a certain spot or area in Diamond or is Ruby actually the only way I'm going to get it on there or how does this go? I already read on the Bulbapedia site and Marriland site about how to get Seedot on Diamond, but I'm not really sure if I understand it that well. Can someone please explain to me how I'm suppose to be able to get Seedot on Diamond?