As far as I know, no matter where your ceremony, you are allowed to bring your own personal floral (either make them, or use an outside vendor.) At the wedding pavilion, you can also use an outside vendor or bring you own decorations, but Disney will not set it up for you, you must designate a family member or friend to do this. If you are having an Epcot ceremony, you must use Disney for everything but personal floral. I'm not sure what the rules are for other ceremony locations.
As far as the reception goes, you can bring you own favors, menus, and place cards anywhere. I think Disney will set out three items for you per person, anything more than that you have to designate someone to do it, or pay Disney to do it. Depending on your reception location, you may or may not be able to bring your own centerpieces and get linens from an outside vendor. Anywhere in the parks, or Atlantic Dance Hall, you must use Disney. Any ballroom you don't have to go through Disney.
HTH!