Were to find venonat on Pokemon Diamond?

actually you can catch venomoth on route 229 using pokeradar

you can get a pokeradar after defeating the elite four and talking to prof Rawn's lab.\

EDIT BY SOMEONE ELSE :this method is way easier than the one listed above After you get the pokeradar use it to catch ditto on route 218 then go to route 229 and catch a venomoth you do not need a poke radar for to catch him or her. Then go to solaceon town and deposit both Pokemon in the daycare. Walk around a little then talk to the old guy up front. He should say your Pokemon produced an egg then walk around with the egg and it should hatch soon added by mikayla895