If you have decided you want a flower girl, and your church will allow it, and it's the right time of year according to the gods of etiquette then at some point you have to decide what kind of flower petals (or leaves or snow or glitter) the cute little moppets are going to cover the church in.
For me, I got myself a free sample from Petal Garden, loved the blue and shamrock options, and got 100 of each. (I never thought of getting real as you can see I am deathly afraid of flowers) There were so many they barely fit into the cute Easter baskets we got for the girls to carry, and they look really cool. Some even have "flaws" to make them look more real.
For some really cheap ones to decorate all over an option is the much used Oriental Trading. There aren't even close to as many color options as others but if you just want white/red/pink you can't do much better.
If you want to go the freeze dried route (much better than regular petals as they won't stain), there are a few other options though they will cost you. (I wonder if you can get an assemble yourself flower petals where they just send you the rose and you pick them off?) Flyboy naturals has flower petals other than just roses (which is nice for those of us not that impressed with roses). Pick a petal doesn't have as many options but is a bit cheaper.
For those who just think rose petals are over done and boring there is the fall option of throwing leaves. And if you're getting married in June you could have your girls toss green leaves of grass clippings for that fresh grass smell.
And we can't forget those winter brides who want a winter wonderland but with central heating. Thanks to the power of science you can easily make some insta-snow without all that iced over freezer and a knife route.
Whatever you decide to have your girl(s) drop just make sure that she follows the exact calculations for the amount of coverage you laid out in a grid design. You may want to invest in some calculus lessons for her.