To answer the original question, there is a florist across from Playero. I would go a day or two ahead to make sure she can get what you have in mind. A second option is to go to Puerto Plata to the Supermercado de Flores, I believe it is called. He is a wholesaler, always has fresh flowers, is a nice guy, and I believe they also do arrangements or he will get someone to do it for you. He is located on the street by the little park near Tropical.
Lindsey