Like a few people have already said, most cats don't react to catnip until they are a little older. It definately depends on the cat though. Two of mine really still don't have much interest in it and the other two love it.
As far as kinds, I grow my own. I just got one of the live plants from Petsmart and planted it in a big planter. I had it outside for the summer and brought it in recently because it started to get cold. During the summer, I also had another catnip plant that was flowering.
I am not sure I agree that catnip without buds is better though. I have bought those catnip bouquets that are all buds for my boys and they really seem to like that more then the catnip that is basically just leaves. They also like the flowers from the plants after they dry a lot more then they like store bought catnip. I guess that might also depend on the cat though.