I am currently in touch with an outside florist: Charming Events. They do floral and pretty much all ceremony/reception decor. I know they have done many Disney weddings before, and I've only heard good things, but I personally haven't had the wedding yet so I can't give my own opinions. I will say that they were quick to respond to all of my questions, were quick to change anything I wanted changed, and super quick about getting the itemized budget to me.
Now that I have them priced, I am going to request the same things from Disney during my planning session and then see which is cheaper and for what. I will say that they require a minimum of $2500 as they devote the whole day to your event. That being said, they do chair covers, linens, lighting, etc. So don't just think that has to come from flowers. Also, Disney now restricts outside vendors from decorating any outside locations. Just a heads up in case you didn't know.