How long ago did you kill Plantera? The bulbs take some time to spawn, in my experience. If you're still not finding bulbs, try creating an artificial underground jungle. It'll also serve useful if you want to farm life fruit. You could also try killing the mechanical bosses again, perhaps something messed up before.
I hope this helps.
EDIT : I just realized that somebody else replied after i started writing so it kind of looks like i'm just repeating what he said. Oops.