One way is to use an action replay, the codes are easily googled.
If you go to Kanto you will find plenty of Oddish.
Oddish
oddish you have to get him at night time
give him a oddish. and a growlithe and a jigglypuff
ill trade you an oddish with sweet sent
an oddish can learn cut in pkmn hg version
Oddish are nocturnal, so catch one when it's past your bedtime on Route 5,6,24,25 or in Illex Forest.
Ilex Forest, Route 24, 25, 5, and 6.
sunkern,oddish,meowth(hard to find there),hoothoot(mostley).
You find Oddish on: Route 5, Route 6, Route 24, Route 25, Ilex Forest Safari Zone
well on Pokemon Gold/Silver/Crystal it's Oddish.
If you are refering to the pokemon that Bill's grandfather wishes he could see, then you want an Oddish.