Unfortunately, the starters are starters, meaning you can only find them at the start of the game, like all starters.
You can always trade a starter from LeafGreen, if you have LeafGreen, or you can breed the starter Pokemon if you have access to the Sevii Islands, if you don't want to get rid of your original starter.
LeafGreen is your only source of trading Kanto Starters, so if you want all the starters, trade over your starter from LeafGreen, then restart, and then choose the last starter you need, then trade it over, and such x3
Hope I helped. :)