While any competent florist is going to produce expert flower arrangements for weddings, keep in mind that there are other options than going straight to the source. Though they often use third-party suppliers, hiring an independent floral designer is an excellent option for couples who want a truly special floral motif for their big day.
Floral designers are more expensive, of course, but they're also artists. Moreover, their services don't always stop with flowers -- "floral designer" is only one of many hats worn by professional wedding decorators who can help tie flowers into an overarching theme and make your wedding an affair to remember.
This isn't to say that flower arrangements crafted by a florist won't be gorgeous -- they're pros, and it shows. But while a florist often juggles dozens of orders simultaneously, floral designers can focus on one client at a time to turn her wedding into a singular work of art.