Keep an eye out for bow ties on Mr Porter. I got my black satin self-tie bow from them.
Warning: Spoiler! (Click to show)
edit: the bow tie was from Brooks Brothers (but off the Mr Porter webstore).
Thanks for this Mr Porter suggestion nabilmust - just had a look and found a Turnbull and Asser silk bow tie which looks quite decent.
Here 'tis: http://www.mrporter.com/product/374256
Although I do like the diamond tip style, although can't find too many of them about.