Rapture:
Yes, I'm inertia from LJ. I have a Lareine fansite too. It might be the one with updating issues... >__<;;;
You can get silk flowers at many crafts stores. Mana cosplayers can probably tell you where to get blue roses in the US; they use them for the "Au Revoir" costume.
I think "favorite flower" means something a little different in this case than what you've been thinking. The profile did say "favorite" in the original Japanese, and that hasn't changed. But, for Lareine, the main thing that "favorite flower" means is what flower you should wave at a concert in order to show support for that member. Whether Emiru really keeps bouquets of pink gerbera in his apartment, we don't know, but if there's a concert happening near his birthday, it's nice for fans to carry pink daisies (instead of the usual sea of blue roses) as a gesture to him.
We don't really give any of them flowers, except for throwing them at the beginning of "Saikai no Hana." Kamijo often takes them from us during concerts, though. I would imagine he's received/taken hundreds of them over the years. o_o
On the other hand, if you can figure out a way to give Kamijo a real blue rose, I'm sure he'd be impressed. That would be really rare! But lately, the answer Kamijo has been giving when he's asked in interviews, "what would you like to receive from fans?" is simply "letters."