I think i would be inclined not to mention the supply availability concerns at all. I would just either be closed for the week or fully booked. Your bride will not understand the logistics and will probably assume you just can't cope with it. She will be out shopping that week and will see flowers all over the place and wonder why you couldn't get them. Bear in mind that whatever reason you give, it will be related to all her friends and aquaintences for the next 6 months.