When I was engaged and trying to figure out my wedding dress I did a lot of research on the best fabric stores.
I found my sash at International Silks and Woolens. They had an awesome selection of ribbons to channel the whole Monique Lhuillier style.
The other fabric store I had some success with was F&S Fabrics. It was a bit pricey, but had a selection of the fabrics I was looking for.
Also, Mood Fabrics has great fabrics as well.
And of course Michael Levine.