I just found the Grand Social the other day. It's a site where designers can list their wares for sale directly to the public - a little like an Australian etsy only for fashion. It has some pretty big names, and it has a couple of dresses that would do nicely for a walk down the aisle.
This, the Petal Dress (deeper links not possible), from Nicola Finetti, is advertised as being suitable for a wedding (I'm starting to secretly suspect that all designers that make a long-ish white dress are shooting for brides, without having to announce themselves as a 'bridal designer').
I really like this Principessa gown by Jayson Brunsdon. Tres chic. There's lots of shorter white dresses in the range (see 'em peeking out behind?), in really interesting shapes too. But this was my favourite.